Demographics: If you are single then... 3 yr -- older students (avg 28) and more even gender balance (50/50) but more men are married, 4 yr -- avg age 23-24, 60-70% female
Academics: 3 yr program can be very tough and if you fail a course you basically become a 4 yr student anyways. Lack of summer internship opps for 3 yr programs (i.e. no USPHS Jr/Sr COSTEP)
Costs: 1 yr of tuition saved plus 1 yr opp cost is worth a lot. Just make sure you find time to network during the 3 yrs to get the job you want
Basically, if you are smart, used to hard work/long hours, already have a significant other and know which field you want to join, go 3 yr.
Want to join USPHS, take a 4 yr and join as SRCOSTEP and be commissioned while in school.
Lots of pros and cons based on what you wish to do.