I run 170's on my SS and 172.5 (what I generally prefer) on my road bike. I don't notice a difference at all swapping between the two since on the roadie I can swap gears and it masks any
difference in arm length.
Re: a 53T front ring. It does not matter that you keep the front ring the same as your other bike. You can run two totally different front rings but as long as your gear inches stays the same, they will feel identical.
Re: Running a compact w/ the 50T ring. That is not a terrible idea. I run a 48x16 and sometimes a 50x16 which is a pretty good gear when I want to work out. a 50t front ring is about as big as I will go when keeping a 16t rear cog. Since you are looking at an ENO, then you are most certianly looking at a WI ENO freewheel. Since 16 is the smallest cog they come in that will be your limiting factor. If you plan on ever running a really big gear, the 110 BCD will also limit you. On my 48x16 I generally average 19-20 mph. You should plan you gear inches according to your strength, your terrain, and the speed you want to maintain (assuming a constant cadence).