I'd avoid Win 8 for at least a year so they can sort the bugs out and get some drivers. Win 7 is looking good at 64 bit.
The problem with the 15" screen lappies is that being designed for portabilty they are not well designed for processing. ie ok for Word and office stuff but for serious graphics processing they are slow, get hot and are power hungry. Aslo their form factor cam limit their connectivity capability and capacity to accept extra drives, ram etc. Quite often no optical drive.
I deploy hundreds per year where I work. The Analysts get big slabs with wide screens and numeric keypads, Sales get the smaller no numeric keypads as they just do documents and comms on theirs. Engineers get similar but emphasis on storage and connectivity with legacy extenders for products problem diagnosis and setup.
Lappies seem to have made it up into the terabyte HD sizes but SSD hasn't gotten there yet. I suspect you will require offline storage for a lot of stuff for a while yet. My new home PC has 2.25 Terabytes but I will still 'archive' older pix files to hot swap drives just to keep the HDs from clogging up. My intended practice is to work on a file collection, derive the output then archive off all the source files just in case I want to rework them one day.
Also remember if you have all your stuff on the lappie and it gets lost\damaged\stolen .....
Theres going to be a compromise somewhere.