It's not the number of speeds but the gear range covered by the drive system that counts. If you are going to ride rough trails with steep hills (like you find in the Southwest) you will need low gears for climbing. That's the reason MTBs are geared lower than road bikes. If you take two bikes
with the same gear range and one is a 10 speed and the other is 27, you have a better chance of finding a comfortable gear for all conditions you face for 27 compared to the 10 speed. You will also find that there is often some overlap in gears so that you really don't get 27 distinctly different gears with the 27 speed. You can use this gear calculator to find out exactly what gears are on each bike if you look at the specifications for the bikes:
Mike Sherman's Bicycle Gear Calculator or
Sheldon Brown's Bicycle Gear Calculator
Rail trails should never be a problem as you will never encounter steep hills on an old rail line. I remember the days when 10 speeds were all you could get so prefer a bike with more choices. You can go faster on the downhills and save your knees on the uphills.