For a good BattleMedic loadout, you should have *Blutsauger - Every hit on an enemy gives you 3 health. It's vampirism baby! *Standard Medigun. Bonk Medics ubercharge them selves and run into battle, distracting enemies or spawn camping. *Standard Bonesaw. It has a REALLY HIGH crit rate, and is an instant kill to the back of an enemy. If you wanted, you could use a solemn vow, since it does more damage, but, it's not nearly as effective.
The Amputator does not do as much damage, but it DOES have that taunt that gives regeneration to your teamates in a huge radius. Deploying that taunt before charging into battle CAN and might just save your life. It has a slightly higher chance to crit, and has a high damage output apon getting a crit. I said to use the regular bonesaw for two reasons: 1. It crits %85 if you hit someone's back. 2. Theres no debuff, meaning it will not harm you in any way.
The Ubersaw, however, I forgot to mention it's upsides in downsides: PRO: When Battle Medic'ing with another player, killing someone with and Ubersaw will gain you full uber. So while your friend ubers you, you gain and uber, and then uber them. Repetitive cycle. PRO: More damage. CON: Less crit chance. CON: Less crit damage. CON: Slower attack time. CON: Getting killed never felt so horrible, because you died right after killing someone, thus dropping a full charge.
In the end, for a pair of Battle Medics, use the Ubersaw. For a single, expert Battle Medic, use the Bonesaw or Amputator. For a single, starter, or mid-skilled Battle Medic, use the Bonesaw.
Now all this that I just spouted is based on FACT and EXPIRENCE. If you have a different style of playing, this might not be too useful for you. So, the Amputator might actually be a good choice.
Heck, I use it all the time anyway. It's good when you're playing Degroot Keep!