Depends what you mean by these two terms.

"Marylebone" used till the 1960s to mean the Metropolitan Borough of St Marylebone: a considerable slug of NW London (http://mapco.net/stmaryle/stmaryle.htm), including Regent's Park, St John's Wood, Lord's cricket ground and the area round Camden Town. You couldn't possibly stroll around it in an afternoon.

Nowadays, the term usually is used for a much tinier area half a mile east and west of Marylebone High St and its continuation, Mandeville Place. Marylebone High St is a fascinating example of modern urban planning: the land's virtually all owned by one estate, which aggressively targets shop tenants it believes will enhance the street's reputation as an upmarket but unblingy, shlock-free (so no Prada or Gucci junk) and interesting place to meander round. The policy works - though it sometimes means targetting tenants who pay relatively low rents. The policy works.

But otherwise, the area is a tasteful but generally boring example of early 19th century speculative building, laid out in a disturbingly grid-like pattern, that's now morphed into pretty much the same collection of posh houses and expensive doctors' surgeries as it was 200 years ago. Couple of minor oddities, but it's just a dullish London neighbourhood - with very, very little greenery

Clerkenwell is infinitely more interesting. Till recently, the term was mostly used for the area immediately round Clerkenwell Green, a major centre of 19th century radicalism (Lenin ran a magazine from there, for example, and Chartist demonstrations in the 1830s were the major stimulus to much economic and political reform). It's now used for a wider area north of the City, which till we started deindustrialising was the centre for hundreds going on thousands of workshops. A lot of them have moved laterally with "their" industry: former typesetting places are now ad agencies, for example.

Though that sounds like the grislily gentrified New Economy quarter of any big city (and a lot of it is just that), the area's got a huge amount of survivals from the past 800 years, during which its role was forever changing - from dependence on a number of monasteries set up there in the 12th century, to Little Italy to its messy modern status. The Visitor Centre used to offer lots of self-guided walks. This really is an area where an afternoon's stroll is vastly improved by a guidebook telling you what to look for, and most London guidebooks are hopeless here: the local history societies, though, have churned out huge amounts.

I say "used to" because it's a few years since I last passed the Visitors Centre, there are disturbingly many different addresses for it on the web, and I can't find any confirmation it's still operating. If no-one can tell you where it is now, invest a few hours googling to produce your own self-guided walk. Ignore the gushing about "vibrant new streetlife" that doesn't just get churned out by crappy guidebooks like this forum's sponsor, but by magazines like Time Out that really ought to know better

I don't know Clerkenwell at all but we enjoyed walking around the Marylebone High Street area. You could get off at the Bond Street tube station, walk up James Street, which eventually leads to Marylebone High Street and look at the shops, cafes and buildings. Along the way, detour slightly to the Wallace Collection which has free entry and is a wonderful small museum. At the end of Marylebone High Street, you could continue to Regents Park. If you have energy and the time, you could continue on to the canal path at the top of the park. Walk along the path to Little Venice then get the tube back.

Or you could do a guided walk in one these areas. This company is very good, we have been on about 10 of their walks. No need to book, just turn up and pay on the day.
www.walks.com

Either way, enjoy your day.
Kay

We recently stayed for a week in Clerkenwell.
Enjoyed exploring around Clerkenwell Green and the St John of Jerusalem priory buildings. Central Market and Charterhouse Street / St Bartholemew's / the original Clerken Well / interesting and historic pubs, etc.

Peter Ackroyd's book "London" has a chapter about the interesting history of the area and explores the linkages between periods of its history.

Which day are you talking about? The neighborhoods are different on Sunday afternoons, Saturday afternoons, etc.

I honestly think it matters less which one people here would pick than it does what made you zero in on these two neighborhoods out of all the others you might have chosen (can't believe these are the last 2 London neighborhoods you've yet to see). It's possible you have a wrong idea of one or both of them when it comes to what you'll get out of strolling around in them on your only free afternoon.

"Which day are you talking about? The neighborhoods are different on Sunday afternoons, Saturday afternoons, etc."

This is true. I wouldn't go to Clerkenwell at the weekend as it's pretty dead. It's vastly more interesting and buzzing during the week, when the local businesses are all open. Some restaurants etc don't bother to open at the weekend as there's no local custom.

Having said that, I agree with Flanner that Clerkenwell is a fascinating, historic and refreshingly tourist free place.

If I were you, I'd take an afternoon London Walks tour-www.walks.com. I've taken 7 of their wals and for the most part you see parts of neighborhoods that are historical and/or significant.

Saturday is the day we'll have free for one of these neighborhoods - from the posts above, it sounds like Clerkenwell would be the more interesting of the 2, but it's dead on Saturday, correct??

I guess there are fewer people about, and all of the professional offices are closed, but there is still plenty to see - the buildings are still there in the interesting street, Museum of London is open, you can wander through to St Pauls, etc. The area has visible traces (and much more than "traces")of the last 2,000 years of continuous occupation, so it's (IMHO) a little fatuous to assume that there'll be nothing to see because you might visit on a Saturday.

For me, visiting when it's less busy is good - I can gaze around in a much more relaxed way, rather than be so anxious about walking into other people or watching out for cars!

Because of a glitch, we now have time to explore both Clerkenwell and Marylebone - on different days. Will Marylebone be lively on a Sunday afternoon? Thanks.

On a Sunday, there'll be lots of people out having brunch in Marylebone and then onto shopping. Don't miss Daunt books on the high street and I'll second the rec for the Wallace Collection. You can take a virtual tour of the area here:

http://www.streetsensation.co.uk/marybone/mh_intro.htm