If only i could write this in my sig as I've said it so many times....
Black rims on cars will look smaller than they actually are, normally an inch smaller, so a 18" will look like a 17", this effect is magnified by the use of a polished rim, as this makes the spokes EVEN smaller and sometimes makes a big wheel look alot smaller...
As its been said, yes Polished rim can be hard to keep clean and can chip, but if you buy a good quality rim, and then coat the rim every week with a alloy wax, they will not corrode and will clean in one wipe of the sponge...
If you do alot of mileage i would not go for a rimmed wheel as that seems to ask for stone chips, but if your keen on keeping your car clean there isnt an issue at all.
I have had my polished rim wheels on now 1 year and i havent got any problems with the rims stone chips or laquer or corrosion on the polished rim.
If i was you Harvey and you was going for 18" i would go for a non rimed split spoke look that you have been looking at, but i wouldnt use BK, Asia Tec or such, as they are the worst wheel make, due to stories of buckling, polished finishes failing and laquer peeling!
Dark grey or black can look good, but if you want the wheels to look alitte bigger, i would stick with light colours with spoke designs to the VERY edge of the alloy.