Neither make a huge difference.

If you're a higher level, go for the seaman cause he gives a boost for fighters apparently. I don't know for sure as I have the medic to keep my loss of sailors to a minimum. Medics do make a difference because they lower the chances and amount of sailors that get killed during the game.

Pilots no longer loose veterans from flight operations, only from crits on the CV.
A medics lessens the amount of sailors lost due to crits.
Cv aren't usually shelled all that much, so a medic has hardly any chance to protect your precious vets, therefore it has become largely useless.
If you want one of those pick the Seaman, he at least has a constant use.

@danita

That is interesting because when my pilots go into the air and get shot down sometimes, they lose rookies and sometimes veterans. BB's tend overkill CV's if they are the last ones especially with HE and AP shells.

In flight they shouldn't loose veterans( officers) cause SDE has turned that off, they do loose rookies and experts. Rookies are obviously worthless and experts you can rather easily regain, so the inflight reduction to crew losses a medic gives is essentially worthless.
As for the infrequent shelling, unless you're winning by the skin of your teeth you shouldn't get shelled as a CV. And if you're loosing you rarely get an opportunity to repair much. Combine that with a SD that's a lot of times not 900 SD and you'll get shelled a lot less than BB that can repair better and have 900 SD.
Given that veteran loss on a BB is already infequent, it's even less on a CV.
This makes the Medic a useless addition really.
Even a Seaman is a better crew member then. A repairman, an unused pilot, engineer for a free support/BO slot or gunners for AA/pHH are all better choices for a Medic as well.

seaman all the way buddy. sure it only gives 3% but hey at high levels 3% is alot. BUT!!!

CAREFUL HOW YOU MAKE IT

a cv seamn needs to have say +12/11 stat fighter.bomber for obvious reasons..

for a bb i would recomend a 11/11/10 rep eng res seaman with 9's in all other areas, boost the sucker too.

dont buy an elite sailor as you wont get the stats you need where you need them.

for example my seaman wa had 11/11 rep engy and the rest where 9 and 10's it was meant for my bb but it works well boosted on my cv too. thoug not ideal for a cv. so roll two seaman one for a cv and one for your bb.

UK lvl120 Seaman for sale. :)