I have a lvl 45 explosive/gun build which means I only have three perks left to choose from.

I had planned for Hit the Deck since I already have the Splash damage perk and Heave Ho! along with the lvl 50 Karma perk. But now that the Mad Bomber perk will soon be up I'm undecided.

The question is pretty much: Since I already have the Splash Damage perk is Hit the Deck a neccessity or is Heave ho! a better choice? Obviously with Heave Ho! the grenades will explode farther away from me thus decreasing the chance of me taking damage in the process while Hit the Deck will decrease the damage instead.

Anyone with experience on the matter?

Hit the deck is the best choice.
You get +25DT when you have explosives equiped.
Also on the Karma perk,go for the good karma perk as that is the most benefical to Demomen.

Well I'm already practically a saint in the game so that's music to my ears.

Can you essentially Grenade/Grenade Launcher snipe in medium-distance with the Heave Ho! perk? How big a difference does it make?

The Grenade Launcher and Mercy/Grenade-MG are probably my favourite wapons in the game so this matters to me.

With the grenade laucher range can be a problem.
You should be able to take out raiders and geckos with ease.
But with deathclaws and bosses you will need to upgrade to a GMG.
If you can,go for Mercy in Dead wind cavern.

But Hit the deck is far more valuable than Heave Ho!

Have you tested Heave Ho!? Are you speaking from experience?

I do have Mercy and emptied Dead Wind Cavern with a fully Upgraded Grenade-MG and it was one of the most exhilarating experiences I've had with the game. I used the deadly Mercy/Fat-man combo in Quarry Junction to make short work of...pretty much anything that moved. It was lot of fun.

Obviously in tight areas such as Caves Hit the Deck might be necessary but how about big open areas?

I have both perks.
With Mercy,you can angle it just right and hit targets a good distance away without heave ho!
The +25DT will allow you to close on distant targets safely and comes in handy for CQC.

Heave Ho, the nicest benefit I noticed was that you can basically "snipe" with Thump-Thump. Thump-Thump has perfect accuracy with Heave Ho (no need to adapt to the iron sights thing and gauge your shots; it just feels perfect from the get-go).

I was leaning towards Hit the Deck but now you made me undecided again lol.

Ok,do you use stealth or stand up combat?

Actually I snipe alot (AMR, Christines Rifle) along with Stealth most of the time. But sometimes you just wanna go Rambo and then I bust out Mercy so it depends.

Well I always assume that Explosives are "Frontline infantry",but thats just me.
Just choose it like this:
Stealth:Heave Ho!
Combat:Hit the Deck!

Well I have a weird Stealthy Sniper/ Explosive build because A) Variation B ) It's fun.

Originally this wasn't a problem but then Obsidian had to be ass-holes and announce the Mad Bomber perk which seems like a must have for explosive chars and the lvl 50 Karma perk is far too useful to disregard which leaves me with only one more spot. There seems to be pros and cons for both but the reason I'm asking is because I can't try them out in practice before I lvl up which is why I'm asking here.

Ok all the information has been stated.
That said however,I would always choose Hit the Deck.
Surviviabilty is more important to me than range.

I'm afraid I cannot be much more help,you will just have to pick.