First. I'll give you some admissions information about DO and MD programs:
-The mean GPA for a DO matriculate in the 2014 cycle was 3.46. The mean MCAT for a DO matriculate in the 2014 cycle was 26.26. (
http://www.aacom.org/docs/default-source/data-and-trends/2011-14-AProfRpt.pdf?sfvrsn=16)
-The mean GPA and MCAT for a MD matriculate in 2014 was 3.69 and 31.4. (
https://www.aamc.org/download/321494/data/factstable17.pdf)
Let's say your GPA after your junior year is on the high end of what you realistically expect and ends up being a 3.4. That means you would approximately have the mean GPA for a DO acceptance and be about 0.3 below the average MD GPA acceptance. A 3.4 is pretty low for MD, but certainly not impossible. If you spend an extra year in undergraduate and raise your GPA to 3.5, that would be good, but only raise your chance of a medical school acceptance by approximately 5% in most cases. (
http://edge.studentdoctor.net/wamc/All_Applicants.png)
A lot of it will depend on your MCAT score. I don't know your ethnicity, so I'll reference the SDN "What are my chances?" success rate chart again for "all applicants" rather than a particular race. With a 3.4 GPA and ignoring all other factors, in order to have a 50% chance to be accepted to an MD school, you'd have to make about a 32 on your MCAT. With a 3.5 GPA and ignoring all other factors, you'd need about a 31 on the MCAT for a 50% chance of being accepted. Do you think you can score at least a 32 on your MCAT? If so, you'd have about a 50% (+/- 5%) chance of getting accepted into an MD program with a 3.4/3.5 GPA. (
http://edge.studentdoctor.net/wamc/All_Applicants.png)
You're deciding between graduating a year early and applying during 2020 cycle (which starts this upcoming May) or staying an extra year to try and raise your GPA and applying during the 2021 cycle (starts in May 2016). If you are leaning toward graduating early, start studying for the 2015 MCAT and try and take your MCAT by this summer. If you are leaning toward waiting an extra year and waiting to apply during the 2021 cycle, you can hold off on the MCAT for now. Also (and I should have addressed this sooner), you can apply to both MD and DO programs. You don't have to pick one or another. In fact, in your case and depending on your MCAT score, applying to both is probably in your best interest.
My advice to you is to wait the extra year. You're a sophomore, and realistically, the GPA gain you could potentially gain in an extra year won't increase your chances by all that much (3.4 to 3.5 is about a 5% increase),
but (and more importantly) I think the extra year could help you get your application as a whole in better order. In terms of the MCAT, if you wait an extra year, you won't have to rush to take the new MCAT in a couple of months, you can see what others think of the new test, allow for more and better study materials to come out about it, and will have more time to prepare for the test (a side note, don't take the MCAT unless you are 100% ready!)
I don't know anything about your ethnicity, extracurricular involvement, volunteering, shadowing, etc, so I ignored those factors and looked purely at GPA and MCAT. Essentially, this is your decision to make, and you do not have to make it right at this moment, but if I had to pick between graduating a year early or staying an extra year with a 3.3 GPA, I'd probably wait a year, but I don't have all the information. Also, waiting an extra year doesn't necessarily mean you have to stay an extra year in undergraduate, either. You could graduate after your third year and attend a post-bacc program for a year while you apply. You could also use that extra year to beef up your extracurriculars by working in a hospital or as an EMT or something in order to get some more money if needed and boost your resume.
A lot of this really depends on how you think you can do on the MCAT. If you think you can rock the MCAT by this summer, get a 32 or above, and apply to medical school by this fall, then by all means I'd save the time and money and graduate early. However, if you are feeling any reservations about the MCAT, then I'd wait until next year to apply to medical school. It will give you an extra year to boost your GPA and your extracurriculars (if neeeded) and a chance to see the new MCAT and better prepare for it.
Also, applying to medical school is a ton of work and a huge time commitment. It might be best to wait the extra year for a variety of factors, some of which I just mentioned previously, and additionally to have more time in order to put together and organize your application and essays better so that you can apply early (which is very advantageous), and also not be as stressed at any interviews. If you decided to graduate a year early, you'd be going to potential interviews this fall while simultaneously graduating a year early and trying to raise your GPA. All that combined with taking the new MCAT this summer while also applying to medical school at the same time seems a little too stressful and chaotic to me, but you could be completely different about that.
So, my advice would be to wait an extra year. I think it would be best for your application for a large number of factors. I'm no expert, but those are my thoughts and advice to you. Post here or message me if you have any other questions or concerns! Thanks! =)