I would guess that the Patch would be more work, because working in assembler is lots harder than writing in C.
The initial creation of OpenTTD would have been a lot of work, though... I know it took at least from June 2002 until early 2003 before something fairly playable existed, whereas TTDPatch had the advantage that the game was already there.
I'm kind of disappointed in OTTD, actually. I would have expected that it would have caught up to Patch TTD as far as features, at which point I imagine everybody would have switched over to using it. But it hasn't particularly in the vehicle-sets area.
Don't be so sure about that, HackyKid has recently been working on newgrf and should soon be applying a number of patches that help implement more newgrf features (eg, short wagons, various callbacks, etc), so it's just a matter of time. PBS has also been in existance for some time and should be committed at some point in the near future.