You don't necessarily need to trim a latex gasket (and you definitely don't need to trim a neoprene gasket--maybe that was a typo?).
I'm sure many can offer suggestions that work best for them. I put 303 on all of my latex gaskets and stretch the neck for a couple of days on something slightly bigger around than my neck. The 303 has the advantage of making the gasket a little slick, so it doesn't pull my hair as much when taking it on or off.
The neck gasket is tight, but I find that once I start paddling I don't notice it at all.
Some people find that trimming is necessary, but if you do this, make sure you get good instructions and do it carefully, or it will rip.
Which drysuit?
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Re: Which drysuit?
All depends on the size of your neck. Cutting is always a last resort. If you put the 303 on the gasket before you do the stretching, it will go on and off easier whatever you're stretching it on. I use gallon bleach jugs for the stretching part and leave the gasket on it for a week.