First and foremost it's a means to get extra cash. I figured if it could be in the beer industry and fit my schedule that's an added bonus. Right now I have no spare time so it doesn't help that I'm driving extra (and spending more on gas).
As an employer - albeit coming from a British cultural perspective so I may be off - I wouldn't begrudge someone halving their commute. At least not too much - yes the timing is pretty crappy just as you're starting to be useful, and recruitment is a pain. But on the flip side you're not *that* important to them if they're paying you less than min wage, it's not like you're the head brewer.
But if the cash is the main thing, then you've got to be a bit ruthless - and I would also not underestimate the real costs of commuting 40 minutes (each way right?), it's not just fuel costs but tyres and everything else.
At the very least you need to try and work out how the money side works out, since that's "first and foremost". What's the full cost of commuting and how much does the shorter commute save (and hence put $x in your pocket, tax-free)? If we're talking two shifts a week, you're saving 4x20 minutes = 1h20m - can you convert that into additional work time and hence $y wages, or at the very least it has some soft value as downtime. Don't underestimate the importance of making debt reduction sustainable, 12+ months of knocking off $500/month works much better than 3 months of living on beans to save $1000/month but then going off the rails for 9 months because you just couldn't keep it up. It's tough working 7 days a week, and a bit of downtime can be an important part of keeping it up.
It depends a bit on how you feel about the non-cash aspects of the current job - the potential for adding to your CV, adding to your brewing experience, just how enjoyable it is to work there. But as you say cash is king here. Still, once you've worked out what $x+$y looks like - which may not be until after the interview - then you can go to your current boss and say - "I hate to do this to you but I've been offered a job with half the commute that is worth ($x + $y) to me, is there a way for you to match that?" Obviously don't do that if it involves a ridiculous increase, but for a small increase it might be worth doing. Depends how you feel about the current place.
As far as your reputation goes - as I say nobody who matters will begrudge you halving your commute. From an employer's perspective, the worst reputation is for theft, but the second-worst is for the kind of flakiness that leaves them in the lurch at the last moment. So don't show up for a shift at 6pm on Friday and announce that you're leaving so they're a man down for the weekend. That's bad.
But try and see it from the POV of your bar manager - if you're in the position to say that you can work a couple of weeks' notice, that helps him a lot. Or maybe if one place is downtown and is busiest with the after-work crowd on a Friday, and the other is more suburban and is busiest on a Saturday then perhaps you could have a week or two of working the busy shift at each. Obviously you don't want to be too "generous" in your offer of notice etc as (assuming you get the offer) your new place will probably want you to start ASAP, but it's worth thinking about this kind of thing.
But it's not compulsory - depends how relations are with the old place.
Also worth pointing out that you are still useful to the old place just because you know your way around the till system, cellar etc and can be thrown into a service without any training. So it can be useful to the manager to have a couple of people like that in the phonebook to draft in for festivals and other big events where they need every pair of hands they can get, or as an emergency replacement when someone's cried off sick 10 minutes before service. So you might want to leave that option open as another way to soften the blow, if you can work it with the new place.
