I use a 16 gauge finish nailer for trim(7/32" stapler for door trim to door frame---what prehung door trim is installed with)

15 gauge heads are HUGE with stained trim, making holes difficult to fill.

MDF trim will dent easily, making 15 gauge holes even bigger.

Thank for the replies. I had read that mdf will split easily with 15g, has anyone had experience with that? I had read mixes reviews for 18g vs 16g vs 15g because of splittiing. I'd love to be able to get away with buying a brad nailer, since they are cheaper but I'm a firm believer that the right tool is priceless. Is it a matter of the thickness of the trim?

You need to be able to shoot a long enough fastener to get through the trim, the wallboard/plaster, and into the framing lumber behind. The longest brads I've seen are 2", and they are problematic in most nailers, with 1-1/2" being a more practical upper limit--not long enough for nailing trim.

I've always used a 15 ga, 2-1/2" nail for trim work-- I don't do it that often and it's easier on my wallet to fill the nail holes than buy another nailer and boxes of nails. I find the 15 ga to be a very useful size in general. The heads are bigger, but the extra holding power is an advantage when you're trying to pull a bowed board into a wall that isn't straight and keep it there.

"I had read that mdf will split easily with 15g, has anyone had experience with that? "

If you get too close to the edge, there is a chance the edge of the MDF piece could fracture if you get within say 1/2" of the edge. But I have to say, I've put a lot of nails through MDF over the years and I'll use 15ga with no worries. I'd recommend staying about 5/8" away from the edge of the board for starters. If you're nailing MDF baseboard and it has a milled profile, I would not nail through the profiled edge at all.

On a test board over a proper substrate, practice good gun alignment. Dial in your pressure. Set the driver depth gauge. Fire away, adjust, when it looks good, move on to your project. Nail straight on to the board, nail on an angle, get close to the edge, see how it performs.

If you get any mushrooming, which you may, you can knock the excess off with a putty knife when you're in the prime and fill stage.I find mushrooming to be board dependent. Sometimes it never happens, sometimes it's every hole.

"I'd love to be able to get away with buying a brad nailer, since they
are cheaper but I'm a firm believer that the right tool is priceless. Is
it a matter of the thickness of the trim?"

Brads on trim can be problematic regarding holding power, in the thinner gauges of brads when driving into harder or thicker materials, you can get wire bending, or brads not fully driven in harder material. The good news is they pull out fairly easily. The bad news is by them pulling out easily, you'll see that they have diminished holding power. Some will make up for the lesser holding power by cross-nailing. The small heads of brads are advantageous for stain trim.

You might be able to cross nail to get the board to suck up to a wavy wall, but there's a possibility that it'll pull away over time and show a gap.

If you can only get one nail gun, perhaps a 16ga might be your best overall gun. If you think you may get two, then perhaps a 15ga and an 18ga. If you do small craft work in natural/stain finishes, then you could add a 23ga pin nailer sometime down the road.

Again, just my opinion. You can get the trim work done with either a 15ga or 16ga. Quite a few people only have one of the other.

Of course later on you'll want to get a palm nailer.

Then you'll then need a pneumatic stapler.

And a flooring nail gun/stapler...

And then you'll need to get a framing gun so you can build an addition to hold all your nail guns.

And a coil roof nailer to roof that addition...

I'm doing 1/2 inch MDF square base right now with a 16 gauge gun and it works just fine. Shooting 2" brads. As long as you put a couple at each stud I don't see holding power being an issue. If there's an awkward section you cold always put a little glue on the back to assist. I'd like to own a 15 gauge gun but I got a great deal on a Home Depot Husky brand set with a 16 gauge finish nailer, 18 gauge brad nailer, and a 18 gauge stapler for ~$75. Husky wouldn't have been my first choice of brand but they have held up great and I don't think I've ever had a malfunction.

I agree that if you're only getting one, get a 16-gauge. But 15, 16, 18, and 23 all have their own best use case and advantages. So like @MongoCT said... you may find yourself wanting the other flavors down the line!