The spec of your laptop/spec of software, and any issues you've had.
Currently, I have following Macs running in the house:
Mac Pro (12-Core 2.66GHz with 48GB RAM, 2010 model) with two 27-inch LED Cinema Displays,
iMac (21.5-inch 3.60GHz Core i5, 2010 model),
MacBook Pro (15-inch Core 2 Duo 2.50GHz, 2008 model)
Matte v. glossy screens?
After using a number of Macs I don't have strong objections on glossy screens.
Matte screen has less reflection, but it still will (depending on your background).
Colour wise, Glossy screen is more vivid.
Processor coping with crunching large 16 bit .tif files?
I don't work on .tif files.
Accuracy of colour representation on screen?
Either matte or glossy, you have to calibrate your monitor. If you are really that professional, you also need to colour calibrate your printer output.
I'm leaning towards a 17" MBP with matte screen but a similarly prices 27" iMac is also in the reckoning
Well, if you don't intend to carry a computer without out get the iMac, you will enjoy the benefit of extra monitor space. (Be advised that 17-inch MacBook Pro is not that lightweight.) However, you can connect an external monitor to the MacBook Pro as well as 27-inch iMac, which means you can hook up a professional-end monitor if you really want to.