What are advantages/disadvantages of one or the other? I assume bigger would be better?

depends on how much you gonna travel with the boat.....if it stays local you can get away with smaller...

You can get far more weight capacity w/15's than 14's.
Standard/good 14" trailer tires will be weight capacity "C" which if I recall is approx. 1660 lbs.
Assuming tandem axle you will have a weight capacity of 6640 lbs. which sounds like a fair until you back out the weight of the trailer. which is gonna be 1500ish probably.
I couldn't figure out why I kept going through tires on the trailer under my old 24' Sonic until I pulled it across a set of scales and just about died! It was over that weight unloaded!
15' tires would have cured it.
If you do much towing (like I do), don't skimp on the trailer. I should start a thread of the crap I see down here! Some of it under brand new, high dollar rigs.

15" gives a much larger weight range, as stated. Go with the largest load capacity tire that you can, it is peace of mind. 14" limits you to 1760lbs a tire, 15" can give you another 500lbs or so depending on the tire.

And they haven't even started telling you about how much faster that smaller diameter tire is spinning at 65mph and how hot your wheel bearings are getting.

That was a thought that popped into my head yesterday when I saw a jetski being pulled down the highway.

why use either one of those ? go 16 inch

I'm in the market for a new trailer and I will not settle for anything else, seen too many peoples weekends or worse ruined by blown tires or burnt up bearings.

That's what is on my Myco, same size as the truck pulling it.

I was only told about 14 or 15 from Chip at Phoenix, I will ask tomorrow if 16" is an option. Has a mag wheel option but doesnt say what size.