I've never participated in 3 gun but I would like to some day in the future. I came into a stupid deal on an 11-87 and while it's not an M1/2, it'll serve a newb like myself just fine.

It's a 28" vent rib barrel. I've got a cart on Brownells with a DMW easyloader, Nordic extended bolt handle and a Nordic mag extension. That's where I'm indecisive. The way I see it I have 2 options:

A) Grab a +6 extension, keep the 28" barrel. The extension will be just a bit short of the barrel. I can still use chokes and revert back to standard capacity for hunting should I need to. However, I don't know how well it'll balance and swing.

B) Grab a +4/5 extension and chop the barrel to 21" or 24". I think it would balance better and look better this way. However, this route kills the option to run a choke and it seems like those are pretty important in the 3 gun world so that's why I'm leaning to option A.

Any advise is much appreciated!

Personally, I prefer a 24" barrel with a +8 (or 2 coupled +4s) extension. A 28" would not be a big deal. The +8 is common in Tac-Ops and is still longer than a 28" barrel. A +6 will be close to even or slightly shorter than the barrel. The +8 (or coupled +4) would allow stuffing a quad load immediately after the start. I have the coupled +4s so I can shorten the gun up for those matches that only allow 9 (8 + 1) total capacity.

Good info, thanks. I'm not too familiar with the different "classes". Are there any matches where having a +6 would disqualify a person?

I'm really tempted to cut the barrel down to 24" but I also don't want to lose the option to change chokes.

Don't cut it. You'll want to be able to change chokes depending on the situation.

I don't know of any that would DQ you but you might get bumped to Open. You'd have to read the ruleset each match operates under. The only one that immediately comes to mind is Blue Ridge.

Best advice I can give. Buy the minimum to function at a local match. Run what you have for now. Try others equipment when you can. Figure out where your gear is holding you back and what works better for YOU. Then spend your money on new gear or modifications. Otherwise, you end up spending time and money on gear that just ends up in a box or being sold off cheap when you could have spent the money on ammo and training and been much farther ahead.

My Benelli M2 match gun is 28".  Yeah it's long, but I don't see how having a 23" barrel with a mag tube sticking 5" past it is much better?

barrel is fine, just go shoot a match

This.

But, so you know . . . barrel length on a shotgun is all about balance and handling. Longer barrels slow down your swing so you can better track flying targets. Not really an issue when shooting 3-gun, I imagine.