you don't have to play it both ways, but might as well, IMO !
I think the dog/under is slightly better, in general.
When I could play these, I'd usually play both ways, but might go 70% to the under, 30% to the over, or any other split I desired.
It's probably better to play both, to reduce variance....if you have an out for it. I wouldn't trust any online book taking it, but would do it with a local dumb enough to take it.
no reason not to take both, but $100 on either is about the same as $50 on both ( long-term expectation).
The trick would be betting an amount ( with the local), that lets you keep playing them, next week, and not kill the guy in one week.
There are lots of other ways to do it.
I'll give one here:
Boxing
Parlay a TKO/KO with under 11.5 ( or whatever) rounds. It's unlikely the Knockout comes in the last 30 seconds of the fight, if it comes.