A 29er will almost certainly leave you behind all but the slowest bikers. A 36er will allow you to reasonably ride with casual bike riders.
A 36er does very well on dirt paths, fire roads and gravel. It can roll over roots and rocks provided there aren’t many drops or much hopping required. Trails that would be a problem are those with steep sections (probably not an issue where you are) or low tree coverage (you’re pretty high up).
you won’t be disappointed with the 36er : ) I can tell you that, it’s my only decent (not free) uni, and it was totally worth it : ) I’ve ridden at Paris mountain, but did have to walk a few sections, and kept to the easier riding trails (except for kanuga!! WOOO best trail ever). Riding light muni is fine. Mounting uphill when you have a UPD is a bear, but can be done with a little practice, learning to hop a little on it may help for mounting on the trails : )
as justtysen said, it’s a big leap in speed, enough that you need the larger wheel for riding with bikes : )
you may want a handlebar too if you’re going to go fast or long distance.
Go for the 36". If you want to ride it down steep twisty trails then I suggest handlebars, a brake and long cranks. With those you’ll be able to handle some pretty technical terrain. A 36" won’t be as nimble as a 24" of course, but if your mountain bike friends can ride it, then you should be able to negotiate that terrain on a 36".