It doesn't really matter as long as your prepared. Maybe get whichever you can find easier or get a better deal on? With mushrooms you'll have a shorter trip which is nice for your first time. On acid you won't be as introspective as with mushrooms, so you might have a better time. I've done acid maybe 14 times and mushrooms twice so I can't speak with as much authority on mushrooms, but they feel a little friendlier and warmer to me, less synthetic, it may be my imagination though. On mushrooms you might be thinking more about the reasons behind things, religion, and things like that moreso than acid. For that reason my reccomendation is acid but it doesn't matter too much.
Have weed and smoke it while you're tripping. It helps if you get nauseous, makes the effects stronger, and all around makes it better.
do some reading on
Erowid about each.
mushrooms lsd