Originally Posted by Retro Grouch
To really make an intelligent decision you need to know what gears you use for the area where you ride and the kind of riding that you do.
You need to know:
1. Easiest hill climb gear.
2. Fastest downhill gear.
3. Favorite flat road gear.
Knowing those requirements, plot out every gear ratio of all of the gearing combinations that you are considering. One of them will reveal itself as being the right choice for you.
The first two are obvious. I put a lot of thought into the flat road gear because I like for that gear to fall in the middle of the cassette so that i can make minor gearing adjustments without having to change chainrings.
+1
Use Sheldon Brown's gear calculator to pair different chainrings with different cassettes. There are other chainring sizes besides the 53-39, 53-42 and 50-34. For the compact, you could go 52-36, 50-36, 50-34, 48-36, 48-34. Triples are often 53-39-30 or 52-42-30. Typical cassettes are 11-23, 12-25, 12-26, 12-27, 12-28, plus plenty of custom options.
Like Retro Grouch said, find the combination that keeps you in one chainring for most of your riding; preferably in the middle of the cassette. That way, little climbs or dipsy-doodles are done with just one or two rear cog shifts instead of changing front rings.
I'm 41, been riding for 1.5 years, and have a 50-34 and 12-25. I've got flats, rolling hills, and plenty of mile+ climbs, some with 10+%
average grades. I can't spin out a 50/12 or 50/13 except on descents and I do greatly appreciate the 34/25 on the climbs. So the compact works for me. I spend 90% of my time in the big ring, only dropping to the small one for big climbs.