Prestige affects who you can negotiate with and where your wrestlers have you in terms of priority (if working for more than one company). It is affected by major signings and losses, show quality compared to current popularity, popularity itself in so much as it affects fed size, ouctomes of regional and national promotional battles, media events (worker deaths, drug use, movies etc) and a number of lesser factors.
Momentum I'm still trying to figure out exactly how it works, but what I've gathered so far is that it majorly can affect match quality and match heat (noted on adv booking). It is affected by wins/losses, match/angle quality, storylines, popularity, morale etc. And I could be wrong, but sometimes what appears to be bad moementum (i.e. 20%) can actually be quite good for a worker with much lower popularity (i.e. 5%) as it indicates an upward trend. That said I've never had anyone last more than 1-2 shows with high momentum on the creative screen and rarely have anyone on that list at all, but that's probably due to my fed size.
Also keep in mind that things like ref, road agent and announcer quality can affect matches and therefore momentum.