Practically, no difference either works pretty much the same thermally. The extra volume of water will only make the system take slightly longer to reach equilibrium, but it's minor.
If you're talking restriction, you can see very minor benefits hydraulically from combined reservoir tops, but it's very minor and almost hard to measure from a pressure/flow variable standpoint. You can see some of this in my MCP35X comparison with this chart:
There is some minor benefits to low restriction combo reservoirs in terms of restriction, but it's very minor.
But...T-lines are also very low in restriction. I have used 3/4" brass Qpex T fittings in the past which leaves almost no restriction at all. It is more work to bleed and may take a few days to completely bleed out, but they work just fine and one less part to mount and take up space.
Downside is looks and slow bleeding.
There are however some negatives as well. You can typically decouple a separate pump better for the ultimate in noise reduction than you can pump/reservoir combos. Pump/res combos can also have vortexing problems unless you use top down inlet tubes or baffle systems to counter the vortexing.
Anyhow, T-lines are fine and in most cases compared to a separated reservoir...less restriction. The bigger difference is slightly more topping off because there isn't an ability to store much water in the upright tube.Edited by Martinm210 - 4/22/11 at 3:26pm