Regarding training, a Family Medicine (FM) doctor and an Internal Medicine (IM) doctor who practices primary care are very similar. Both receive 3 years of residency training, in both the hospital and outpatient.
FM docs are also trained to work with kids. All things equal, IM docs will thus have proportionally more experience with adults. More generally, as said by other answerers, IM docs will usually have more experience with particularly sick adults.
The truth is, both are equally qualified to provide primary care for a healthy young adult male. It's hard to say that the quality of your care will be better from one or the other.
That said, the demographics of FM docs and IM docs are different. I don't wish to stereotype, so this mostly helps predict what you'll find if you're picking one at random - I wouldn't apply it to individuals. To repeat: I have the utmost respect for both professions, and I intend this to be as neutral as possible.
- IM docs are more likely to be young
- IM docs are more likely to have graduated from a foreign medical school, and/or to be a foreign national.
- 15% of FM doctors have a DO instead of an MD (as opposed to 5% for IM). It's controversial if there is a difference in quality, but DOs receive more training in massage and other body manipulations.
I have no source for this, but I'm gonna cite it as semi-common knowledge. I thought about leaving it off but it's a big deal to some people:
- Most physicians are at least somewhat disdainful of "alternative" therapies (at least the ones with insufficient research). This is still true in FM, but it's my experience that the rare doctor who does have an interest there is more likely to end up in FM. If you're big into alternative medicine I would lean towards FM, if you're not then don't let it scare you away. (Please read this paragraph as neutrally as possible; I don't want to start a derailing debate)
Conclusion: your care likely won't suffer either way, but you'll be choosing from a slightly different pool of personalities and demographics. Naturally, all of these are generalizations, individuals all differ.