Don't forget that internships aren't only good for the airline that you interned at, at least when thinking about getting hired at the regionals. Having an internship proves that you're able to function productively in the 121 world and have knowledge of 121 operations (regardless of the department). If the choice is to do an XJT internship or a major internship, take the major, but if the choice is any internship or no internship, definitely take the internship. You WON'T have the hours to set yourself apart from the pack for a long time (since the excess pilots have 121 turbine time right now, for the most part), so you have to find ways to set yourself apart in a different way. An internship is the perfect way to put yourself above the fray in terms of your fellow graduates and into at least the same ballpark as furloughed 121 guys and gals. Don't forget also that you'll have an opportunity to use the sims at XJT, which, while not logable, is at least something that you can bring up during your future interviews. How do they know you'll make it through training? You've already been immersed in a 121 environment (and hopefully excelled; bring letters of recommendation), and even had the opportunity to have a jump start by getting, say, 20 hours in a ERJ sim. Along with the fact that there's no 121 job waiting for you when you graduate anyway (well, probably not, sorry), there's no rush to graduate right now. Sure, you may be disappointed and never get hired with XJT as a pilot, but an internship will only help you, where ever you go, while not taking an internship will only hurt you in the end. It's like taking a free $100 towards buying a car. It might not be what you need to get the car, but it's pointless to turn the $100 down. While the unpaid part definitely sucks, you could probably find something that you could do casually on the side (like CFIing!) that would at least help pay the bills.

I say go for it unless you have to promise your firstborn to your bank in order to afford it.