I've always used either a single chain ring (with single sprocket) or a triple (with a load of sprockets) on MTBs. Looking at getting a new MTB....but it only comes in 2x10. Any thoughts on how much of a compromise this is - have you used a double on an MTB...how did you get on with it? Do you find that you are faffing around having to change the front all the time rather than making quicker changes to the rear?
My first MTB was 21 speed (3x7) and rode it with no problems. So 20 speed isn't that much different. Biggest difference must be like 50/34 compacts where, when changing from large ring to small, makes a very big jump in gearing.
The easiest way to find something lost around the house is to buy a replacement.
You could look at the gears you most commonly use on your triple and then the usable gears on the mountain bike and work out whether you'd have to be changing regularly in normal riding. With wide range cassettes It probably isn't the issue it was with fewer sprockets and close ratios.