What do you want to see in each city, and how much time will that take? It's all about you.

I really havent done much research, as we are just in the early planning stages, but i guess the highlights. And we saw the highlights of London in 1 day ( yes it was rushed, so id say a min of 2 days to see highlights)

I'm planning a trip to Barcelona for 8 days. One day trip to Montserrat. I'm not sure 8 days is enough time in Barcelona.

Eight days is more than enought unless you actually want to enjoy the city. Having been several times, I think you need to apend at least one week to get a taste of the city.

I would recommend 5 days in BCN. There is plenty to do and see in Barcelona with 5 days, and I would keep the option of a daytrip as well. We spent 5 nights in Barcelona and LOVED it! Here is a link to my trip report:
http://www.fromhometoroam.com/2012/06/barcelona/

When are you planning to go? Huge climatic differences in Spain, and then you have all the local festivals that can be a great introduction to the food, wine, history and culture or something you might want to completely avoid. Spanish festivals are serious stuff, and whole cities are turned upside down 24/7. Here are some of the most famous: http://www.spanish-fiestas.com/festivals/

Barcelona festivals (the Mercè festival in September is the major): http://www.timeout.com/barcelona/features/128/festivals-events-in-barcelona

Madrid festivals (San Isidro in mid May is the major): http://www.timeout.com/madrid/features/212/festivals-events-in-madrid

I recommend 5 days in Barcelona, too, with a possible day trip to either Girona or Montserrat.

Well, to offer a different opinion here... I would go to Madrid. And then , if you get "bored" in that exciting city, you could do any of the proverbial day trips to Segovia and/or Toledo.

I've never been enamoured of Barcelona, as so many appear to be. As everything, it's really a matter of personal opinion and preference.

Oh no. I see that the La Merce festival is right in the middle of our trip this September, and if we go, it would mean rerouting our trip entirely and / or doing more back and forth from the Girona province where we wanted to spend a week!

Anyone gone to this? Is it worth making the extra effort to be in Barcelona for it? We generally don't like being in big crowds.

This is the one where they have giant puppet parades and about four or five days of street festivities to mark the "feast of the day of Our Lady of Mercy." It really does look fun. Just wish it was better suited for our plans!

This is how we did our Spain (and Portugal trip in 12 days)--
Arrived Barcelona, spent a few days here to see must sees and did a day trip to Montserrat (buy a combo train/cable car ticket at a FGC station because it saves you the long line at Montserrat Aeri to purchase the cc ticket). If you wish to see the acapella boys choir make sure you are at the top already, inside basilica at 1 PM since it only happens once a day after the mid-day mass. Took speed train to Madrid and spent some days for city must sees and did day trips to Toledo and Segovia (separate days for each). We then proceeded to Lisbon from there via an overnight train to spend the rest of our vacation with a day trip to Sintra. It was not hurried and we got to see alot since we planned each step-- not bad for our first time there.

If you don't like crowds, I would have gone to fabulous Madrid instead of staying in Barcelona during La Mercé. Madrid would have been my first choice anyhow even if I like Barcelona very much.

But nobody knows better how to make a great festival than the Spanish, and the Mercé festival in Barcelona ("The last blast of summer before the autumn starts") would be no exception. Could be great fun, and you will always find some quieter parts in town. Maybe stay out of the more touristy areas, and go the center for the different events. http://www.bcn.cat/merce/en/index.shtml

I didn't explain myself well-- we will be in Barcelona just before La Merce and then we have planned to head north for eight days. I was trying to figure out if we should adjust our trip to go north, then return to Barcelona, then head north again.

It's moot now, because we have decided to keep our itinerary as it is. There is a fabulous writeup on La Merce on Tripadvisor, with alot of specifics about the festival. It looks wonderful, but a number of the events require getting there hours early to ensure a spot, and that's not really our type of thing. Had we already planned to be in Barcelona during that time, I am sure that taking part in the festivities would have been fantastic, as there is a ton of stuff going on.

Anyway, the human towers -- the Castellers -- sound so amazing, and apparently these groups perform around Catalonia for several months. I think there is one in Figueres while we will be in that region.

I wanted to add that I will be in Madrid before heading to Barcelona.

OP is trying to decide between the two cities; i was just trying to figure out how that festival would affect my itinerary.

And now, I return this thread to its regularly scheduled programming: Madrid or Barcelona, or Madrid AND Barcelona.

We are going in May! I think we will do BCN and a day trip
What are the must sees of BCN? We like history, churches, experiencing the culture etc
What would the best day trip be?

What are the must sees of Barcelona? I have 28 of them:

Sant Pere and La Ribera
** Palau Dalmases
** Palau de la Música Catalana
** Museu Picasso

Eixample
** Casa Milà (La Pedrera)
** Casa Golferichs
** Casa Domènech i Estapà
** Casa de les Punxes (House of the Spikes)
** Casa de la Papallona
** Temple Expiatori de la Sagrada Família
** Casa Àsia-Palau Baró de Quadras
** Casa Calvet
** Hospital de Sant Pau

Upper Barcelona
** Monestir de Pedralbes
** Palau Reial de Pedralbes (Royal Palace of Pedralbes)
**Torre Bellesguard

Barri Gotic
** Plaça de Garriga Bachs

Montjuïc
**Museu Nacional d'Art de Catalunya (MNAC, Catalonian National Museum of Art)
** Fundació Miró
** Magic Fountain
The magic fountain does not operate on Monday, Tuesday or Wednesday.
http://www.bcn.cat/parcsijardins/fonts/EN/magica.html

Rambla
** Casa Bruno Quadros
** Font de Canaletes
** Gran Teatre del Liceu
** La Boqueria
** Palau Güell
** Sant Agustí

La Ciutadella and Barceloneta
** Farmacia Saim

Gracia
** Casa Vicens
** Casa-Museu Gaudí
** Park Güell

The one day trip I have planned is to Montserrat.