Its not very difficult to do a swap from M20 to M50 but it definately involves a lot more work. it uses a totally different wiring harness and electronics and obviously it will cost more since you will need to get a full half cut to do the conversion.
M20B20 to M20B25 is a very straight forward swap, just yank out the 2.0 and drop in the 2.5 and it will run. The M20's share a lot of common stuff and you can use back major items like power steering pumps, alternators, AFM, etc so it will be cheaper to convert.
If you are going from a M20B20 to B25 auto, then the 30+ bhp gain in performance will be minimal. Infact over time you'll soon feel as if the auto is still so sluggish even with a 2.5. However if you are using a manual then the diff in performance will be much more noticeable and a whole lot more fun to drive.
Of course if you can afford it do the M50 conversion lah, the extra bhp, torque and smoothness is worth it but if you are on a budget then the M20B25 manual will be a good compromise. I heard lot of forummers say the M20's are cheaper and easier to maintain too. So its your call.. just congtributing my 2 kupangs..... 
