Loyalty to the union means nothing. It's not an issue at all. You pay them. They don't pay you. Whatever shop you land in, the union is there, and you pay dues. In your case, you've indicated it's the same union (same local?), but it really doesn't matter. There are no brownie points for staying any longer, regardless of what morale pins you might get for being with them a little longer.

Loyalty to the airline is really irrelevant. You're a seniority number. There are no points for "loyalty." You're not planning on staying long term at either one, so loyalty is not remotely relevant.

There's a perception, quite false, that a pilot flying for a "ULCC" is somehow closer to god than the pilot at the regional, and thus more likely to be blessed by the sacred minions at the legacy lands of nirvana. Not so. The legacy carrier isn't impressed by an airbus type. They're not impressed by a dispatcher certificate, or other certification; it's not like they're hiring you because you'd be a great fall-back to their dispatch if things don't work out. They're hiring you as a pilot. If you go to the legacy, you'll fly what they tell you to fly, and you'll go through the training regardless of what you've flown before. If you have no type in their equipment, it doesn't matter, because they'll type you. If you hold a type for what they fly, then you've still got to undergo the training, anyway.

The recruiter wants to know if you will be a good investment. They're going to invest in you the same amount of money regardless of where you come from, and regardless of your type rating. The only place that loyalty will make a difference, and it's not really loyalty but simply being present, is flow, and that may be available, or may not.

Given that there are a LOT of regional pilots applying to the legacy carriers, one might say that what you need is a way to distinguish yourself. An added type rating might be a way to do that, but it's not as flashy as all that (and chips). I've got a wallet full of certificates and a few types along the way, and that plus a few bucks won't buy a cup of coffee. It doesn't impress anybody. In my opinion, what will distinguish you isn't checking more boxes ("ULCC," airbus type, etc), but advancing yourself. Dispatcher, vs. captain. First Officer, vs. Captain. Captain, then while waiting, perhaps check airman. Check airman at a regional is a worth a lot more than First Officer in an airbus at a poverty carrier. I don't think anyone will care if you showed "loyalty" to your current operator, or to the regional. Nobody will fault you if you do either one. I don't think either will hold you back, but given the higher wages at the regional, and returning to a captain position, personally the regional would probably sound better.

Ask yourself this: the business and the decisions of where to go and what to do are always made with an eye to where you'll be and what you'll be doing if the music quits. You don't want to be somewhere and be furloughed, obviously, but we don't really control that (or anything else)...but ask yourself what happens if the music stops and you have to stay in your seat. If the industry slows to a standstill and the legacies stop absorbing new hires, if the plan to move on doesn't pan out for the forseeable future, would you be better off at the regional, or the low cost carrier? If you took the airbus job and then got stuck, would you regret not going back to the captain position, in base, at the regional?

It's great to add a type; to stay in the practice of learning a new aircraft and doing that dance (it gets old when you get a little older: trust me), and it doesn't hurt to have more squiggles on the little plastic card in your wallet or on your resume. It doesn't really help a lot, either, unless you need to practice training for a new type that you may or may not use much (I'm the patron saint of useless type ratings...aircraft nobody flies any more and that will never get used again. They're memories, but it's hard to buy groceries on memories). You can buy groceries with a higher wage and money saved by living in base, though...so there's that.

Practical, vs. gee-whiz, I guess.