'Venmo' itself has become a verb rather than just a service so I doubt either of them have the necessary momentum to take these services to anything more than nominal levels of volume relative to the market lead
I've literally never heard of Venmo before now
I know for me personally I'm far more likely to say "you get it next time" than bill my friends for some share of an outing. If it's sufficiently expensive that this doesn't make sense I'd just split the check. If I was younger and still a student I can absolutely see the appeal though.