I downloaded an iPhone app called Shiftit! to help me understand the gears, rpm and speeds.
I have a compact ultegra 6700 50/34 and 11-28 cassette
I find myself often in 34/13 (rpm 90 - 18 mph), my question is should I switch be in the 50/19 instead of the small clog?
Does it make a difference?
Does the angle of the chain on the 34/13 pose a problem?
They are almost the same gear. The 19 and the 50 are probably more aligned (by a bit at least). Thus, there would be a bit less wear/friction in the 19/50 (it would be slightly more efficient). The wear will also be less for a larger cog/chain-ring.
If you are likely to need to shift to lower gears (for upcoming hills), then using 34/13 might be preferable. If you are likely to shift to higher gears, then 50/19 would be preferable.