AAMC: 508 127 C/P|125 CARS|128 B/B|128 P/S
KAPLAN: FL3 505, FL2 502, FL1 497
Was 508 your best AAMC FL or was it the only AAMC FL that you took?
The reason I ask is because it's generally a good idea to take all the AAMC FL's before taking the real MCAT. If you didn't take all the AAMC FL's and/or if you positively believe you can raise your score with a couple months of study, then by all means try again! This time around be sure to work through all the practice resources the AAMC offers, including FLs, QPacks, and the CARS Diagnostic. And remember to review your practice passages -- try to figure out
exactly where you went wrong on each question (was there a gap in your content knowledge, were you confused by the passage, etc.). Then once you've diagnosed the issue, take action to fix it (review a concept, decide on a strategy to help you read better, etc). This cycle of "practice-review-practice-review" is ultimately the best way to improve at the MCAT, at least IMO.
Alternatively, if you think that the 506-508 range is pretty predictive of your potential, you could go ahead and apply to MD programs without retaking. That said, if you choose this route, I would definitely make sure to apply to DO schools as well, just to maximize your chances of getting in somewhere.
Of course, this decision also depends on a hundred other non-MCAT factors, factors only you are aware of. So get some advice from family, friends, this forum, and then... go with you gut. There probably is no "right" answer here, so just view your decision and its outcomes as learning process.
Good luck!