In theory, the MTB derailleur would work better, but I'd suggest you save your dollars and keep your 105. If your chain falls off the small ring sometimes, just lower it a bit and turn it sideways (a bit) so that the bottom part sticks out a bit more than the top part.
For your information, I'm using 44-34-22 with the original 105 derailleur and bar-end shifters. After some 15 000-20 000 km, no problems to report. The only slow shift is when I upshift from the granny to the middle ring.