Web development, in most cases, doesn’t really require much from your machine. Even if you are spending most of your time doing UI and front end designs in Photoshop even the lowest spec’d mac you have mentioned would be enough. The machines these days are capable of providing enough grunt.
2GB of ram is pretty standard - but if you do go Imac make sure you bump up the ram to at least 2GB.
The more ram the more programs you can have open without the machine taking a hit. In saying that I find the memory management system in OSx far superior to XP/Vista so having a whole slew of programs open rarely slows thing down.
For web dev work, unless you’re looking at any form of video/animation/rendering, I would say that the Mac Pro would be overkill. Even if you had a one of video jo the MBP or Imac would have more than enough power to see you through.
I guess it comes down to the portability requirements. I don’t like working from a laptop - even if I plug in keyboard/monitors etc - so I have the MBP when I’m on the road and seeing clients and have the Mac Pro at home where I do most of my work (I am studying Animation and I do need 4 core/8gb of ram for that work 😊
I’d probably go the MBP (wait for the next version - possibly rumored to be coming out tomorrow - if not they aren’t far off). You’ll have the flexibility to plug in monitor and keyboard if you want to work at home and then you have everything in one compact (and beautiful) enclosure for the road.
Don’t forget Mamp Pro (http://www.mamp.info/en/mamp-pro/) for you local dev setup.