You know, I really think it would be great if there was some sort of database where we could draw in users experiences with different rigs so that we could compile a list. I really don't think it's helpful when people ask if this will work or that will work and someone chimes in with "oh you need this" and then lists the top of the line computer, because there is always a "budget to overkill" ratio for very user.
For instance I use an imac with 4 gigs of ram and it works perfectly for my needs. Now I don't do alot of projects where I track 8 or more instruments in a single pass so I can't really speak to that issue. I can say that I am able to use a substantial amount of plugs at mixdown and a final track count of 24 to 32 tracks without a hiccup. So if for instance this is how you work maybe you would prefer to get the imac and take advantage of the larger display vs the macbook pro.
Not everyone needs a macbook pro or a mac pro ... in fact for alot of us that might be overkill and the extra money could be spent on mics, hard drives, interfaces, the mortgage...If you have a band set up with full drums individually mic'd and you are going to try to record 12 to 16 tracks in a single pass.. maybe you need more than an imac, I don't know I've never tried that OR perhaps if your final track count is into the hundreds and you have countless plugs and software synths, I'm sure you will need the most power you can get.
Perhaps I should take the initiative and start a thread where people post their methods of work, the set-up they use to get it done and then I can compile a bit of a final list ... ie: if you work like this you can get away with this set up... when you get into this kind of track count it starts to bog down but people are managing with this... etc...
That way when people post questions like this there will be a way for them to strategically figure out if a certain computer, or amount of ram, or interface, drive etc ... will actually be fine or if they are going to encounter trouble...
Without building from each others experiences none of us can really say. Unless we have actually owned every system and tracked what it can do .... just a thought