I haven't seen it 'up' long enough. I'm sure that the darker the siding, the more expansion and contraction. As far as the seams go, the seams should NOT be seen when you're standing at the front door. The seams are installed 'away' from the line of site. In other words, you should see the seams when you're approaching the front door from the BACK of the house.
I have white vinyl and the seams are, basically, unnoticible. I say go w/the seams.
PS: Make sure you have a flat sheathing. If you're siding over any type of shingle, remove them first. Otherwise you can void your warranty! If you need more info, e-mail me directly.