Speed-drive makes the chainring spin at a higher rate. This means that a 38 tooth chainring, when speed-drive is engaged, performs like a 63 tooth chainring. If you have a 44 tooth chainring, with speed-drive engaged this chainring would equal a huge 73 tooth chainring.
That's why speed-drive is often used for fast bikes like recumbents, or folding bikes, where it compensates for the small rear wheel.
Mountain-drive reduces speeds by a factor of 2. 5. With mountain-drive there is normally no overlap between gears.
Mountain-drive enables you to climb steepest hills with your bike. If possible, take a slightly bigger chainring (2 or 4 teeth larger) to get higher gears as well as lower ones.