Perrotfish has finally convinced me: an SMP is more effective than additional undergrad coursework. Do try to get into a good SMP (good means its grads get into med school). Do have all your app assets tidy before you start the SMP.
Compare:
A) For each additional year of full time undergrad, you'll see no better than a .2 GPA improvement, most likely closer to 0.1. On the other side of more undergrad, unless you do 4 more years, you won't have a competitive GPA (please confirm using actual math). You will have the much-lauded upward trend. Nobody will see that trend if your cumulative GPAs are low - you won't make it to the eyeball round except maybe in your home state.
B) In a good SMP, you get a med school transcript. Schools that recognize SMPs (because SMP grads get in and do well there) will pay attention to the SMP despite your cuGPAs. Also in a good SMP, you get same-year support for med school apps through letters from the program dean.
Best of luck to you.