I think with shelter work, it's important to make sure you're really ready for the potential for long hours, little pay, and for things to end up much more stressful than you thought they would. I was often pretty stressed and depressed when I worked in the animal shelter. I was also often really happy and thought the job was very rewarding, but I just wanted to point out that it could be a lot less of the whole rewarding puppies and rainbows thing than you think going in. If I were you, I'd see about talking to some staff members about what the working conditions are REALLY like. I think sheltering is definitely a situation where good management/leadership will make or break you as far as happiness at work goes.
THIS. I've worked at my shelter for about a year. We work holidays, long hours, and don't get overtime (because at a non-profit trying to save money during today's economy, why would they pay us overtime?). Everyday we get ungrateful people, stupid customers with children who poke their fingers in cages, ignorant people dropping off animals for no real reason, it's mostly a thankless business. For the past year I have absolutely loved my job, my great staff and management made the bad days worth it and I've always been eager to come into work despite the hours and low pay ($8/hr here). However we recently changed hands and our new manager is awful. I'm not going into too much detail here (not trying to get dooced) but every bad thing that could have happened, has happened.
Through my job I've gotten experience with ferrets, chinchillas, goats, pigs, geese, snakes, rats/mice, guinea pigs, and of course cats and dogs. I can now draw blood, give microchips and injections, restrain animals, and can humanely euthanize. I've gotten about 1200 hours of straight animal experience and have at least 2 amazing references to take with me. I hate that I might be leaving the job that turned me on to vet school, but when shelter work is awful, you're going to hate everyday and it will stress you out when you're not there.
You should really research your shelter. Talk a lot with the people that currently work there. If it's a great place, it can be really rewarding and teach you a ton, but if you're going to be fighting with management that treats you like a janitor or time limits on adoptable animals, you might find it a potentially bad experience, especially after giving up your last semester of college. I think a good job is worth more than that GPA boost, but I think you should be very thorough when researching the shelter. I would be super honest with employees (the ones who will be your coworkers and your superiors). If you're giving up so much to do this job, you should know exactly what you're getting into.
Sorry I wrote a novel, it's just a situation that's pretty near and dear to me right now-- if you've got suggestions on how to deal with incompetent shelter management, I'd love to hear it!