Both pak64 and pak128 are good for multiplayer, especially when pay for distance is set to 0 (between stops). Pak64 is more newbie friendly and generally feels more polished than 128 which has several easily noticed flaws. Pak128 is significantly harder, to the point that people noticeably try to exploit game mechanics more but it does offer potentially more to the player with its elevated ways and larger convoy selection range.
Reality is that Simutrans lacks a very well "polished" pakset at the moment. There are dozens of issues with pak64 and 128 that I could sprout off the top of my head. Be it bugs, balance issues, usability or simply missing features.