Both good choices but it depends on what you are playing. The I would also add the Yamaha TRB 6, the first run of the TRB series - the ones with the stacked single coils not the later ones with the soapbars.
The MTD is a little stale without any mods. Structurally its fine (well the one i tried was), actions awesome low, zero fret buzz, feels really good and the neck is awesome. But the sound wasn't worth the price tag

. Not to say its bad, it just didn't have enough "meat" on it. That said it would be perfect for solo bass stuff but wouldn't cut it (again this is all my opinion) for Rock and such. It could work for jazz with a little eq-tweaking.
I owned a Yamaha TRB 6 for a few months and i loved it. It could do everything well. At home i used it for whatever; Dream Theater to Jamiroquai. And school I used it in the Jazz band as my main bass and those were the only compliments i got on how my bass sounded, lol. It was called a "jazz band" but we played pop, funk, modern gospel, and some fusion. The only complaint i had was that even though it had a 3 band EQ there was very little you could do to get a different sound out of it - similar to a P Bass it doesn't have many sounds but what it does have will work for just about anything. They tend to be a little heavy (mine was 12lbs) but they more than make up for it with their sound and playability.
As for the Carvin LB-76 i have a LB-76WP (that i am selling if you are interested

. And its only because i've found another 6er that i REALLY want and have no need for more than one) which is essentially the same bass with piezo pickups. This is far and away the most tonally versatile bass of the bunch. That could be due, in part, to the piezos but even without them i think the statement stands. There hasn't been a situation where i said to myself "I can't find a sound for this". The necks are awesome on these and they are really light (mine is 9.5ish lbs) but don't make you feels as if its fragile or something like that. That may not matter to you but i've had 2 basses that i was afraid to slap on because they were so light and i thought something would happen if i beat on them too hard - call me paranoid

. Strings spacing isn't quite as big as the MTD or the Yamaha but its not so small that it takes forever to acclimate your slap style to them or that people with big hands would have trouble navigating the fingerboard. The sound has enough "growl" (not the aggressive Geddy growl, but more along the lines of Stu Zender just not as much) fit just about any style without being overwhelming. All that aside, if you are a person who likes simple controls and doesn't want to have to worry about tweaking the EQ to find a sound for you this is not the bass for you. It took me 30mins (maybe i'm just slow?) to dial in my "ideal tone" but it was worth it.
Anywho, if anything i suggest that you swap the Yamaha in for the MTD and decide between that and the Carvin. You can't go wrong with either really.
EDIT: I just noticed that i used "awesome" to describe the necks of all the basses. Blame that on my lazy vocabulary but the necks on all of these basses are... well... awesome, lol.