I've always thought that if I got the chance it would be "neat" to take the train through the Chunnel. I'm in aerospace, I enjoy engineering feats and I agree that the chunnel is a man-made wonder.

With all of that said - at the end of the day, you're on a train in a tunnel. Not a lot in the way of atmosphere I imagine.

Removing speed as a factor:

Is there a measurable difference between crossing the channel by ferry, or by taking the Chunnel?

>>measurable difference<< - you said it yourself, it's one of speed, not only for the specific journey, but also for the end-to-end connections (assuming you're planning to travel between London and another European city).

It might be cheaper if you're travelling by car (I've no idea) to go by ferry rather than the shuttle train through the tunnel.

But it comes down to whether you want to get from A to B or get an experience. As you say, the experience of travelling through a tunnel is the same whatever's on top of it. It's not as though they'd built a transparent observation roof.

Besides speed, the big difference is that on stormy days, there is either no ferry or else vomit all over the deck and corridors.

Thanks Patrick and Kerouac
(lol @ Kerouac also )

Yes, btw we'd be crossing to go to Paris.

Eurostar trains now take about two and a quarter hours between Paris and London (and no more than 20 minutes is spent in the tunnel).
By train and ship, there are no longer special trains directly connecting with ships. It's a good hour and a half from London to Dover, then a bus to the ferry terminal, another bus to the ship, an hour and a half on the ship, a bus to Calais ferry terminal, another bus to Calais station, then two to three hours to Paris, often with a change of trains en route.
The train/ship connection can be much cheaper. To get a reasonable fare with Eurostar, you need to book well in advance. If you turn up at the station and ask for a one-way ticket, you will pay two or three times the total fare for for the combined journey by train and ship where there is no price advantage to buying tickets in advance.

you are only planning 3 days in London and 3 days in Paris - you simply don't have the time for anything but the Eurostar.

Even if you had the time though, the Eurostar is the best way to travel between central London/central Paris

Thanks Geoff and Janis.

Geoff - excellent details on what a ferry crossing entails, thanks much!

I would guess that departing from sparklingly refurbished London St Pancras and arriving at splendid Paris Gare du Nord adds considerably in count of travelling-in-style...

And I will add that the whole Gare du Nord area has been refurbished, for those people who always shudder at the idea of that neighborhood.