I've been comparing the two of these this week as well, and I think I'm going to actually stick with the Martial Gun. I've taken a liking to using Detonator for the most part, then using Slug every 5 minutes when Sharpshot is up.
I played around eating Coeurl Subs, and the hit in melee accuracy that I took from not using Sushi was noticeable enough that I didn't want to hold to 200+ tp that often. If you fire off WS at 100% as often as possible and actually land all of them your damage output will be much higher than holding to 200+ often will. And despite the lower base damage, Detonator at 100% is a big difference on the 2 guns from what I've seen so far, while the Slug damage really isn't that different. Detonator will never do as much damage as Slug, that's impossible b/c of Slug's high fTP mod, but a landed Detonator will always do more damage than a missed Slug, and we both now how Slug acts at 100% sometimes.
From what I understand, aside from scaling well with TP, Detonator also received a hidden RAtk boost when they improved it(although it may be an 'ignore defense' boost like Split Shot instead'), and your RAtk/Def ratio is one of the hardest things to raise as COR. Detonator is a lot more useful than missing Slugs often, especially in a party where you're eating meat and using rolls other than Hunter's.
I want to play around more before making a final decision, but at the moment I'm definitely leaning towards the Martial Gun.
Edit: I decided to put some hypothetical #'s to this in a COR guide that I'm working on, just for the sake of comparison...here's the excerpt:
Martial Gun vs. Coffinmaker
This part I'm not going to get super mathy on, really it's common sense if you look at the formula. Let's say your pDIF is at 2.0. The Martial Gun has 39 D, Coffinmaker has 40. The WSD formula is basically Base WS damage * fTP * pDIF...if you have 5.1 fTP and 3.0 pDIF, that means you have Base WS damage * 10.2. Logic follows that adding 1 D will net you a 10 damage boost for Slug Shot. I can't really tell you if the accuracy boost from an extra 100% TP is greater than the +9 from Coffinmaker, but they seem pretty close.
For Detonator, however, Martial Gun will out damage Coffinmaker when both weapons are at less than 300% TP, and by more than 10 in most cases. This is because Detonator gets higher fTP values tiered at every 100% TP, and the TP Bonus on the Martial Gun will make it act as if it has 100% more TP than it really does. If we assume a Base WS Damage value that includes fSTR2 of 9 and WSC of 18, and keep the 2.0 pDIF, let's see the differences:
Coffinmaker @ 100% TP- WSD = 137 * 2.10 * 2.0
WSD = 574~
Martial Gun @ 100% TP- WSD = 136 * 2.85 * 2.0
WSD = 774~
Coffinmaker @ 200% TP- WSD = 137 * 2.85 * 2.0
WSD = 780~
Martial Gun @ 200% TP- WSD = 136 * 3.10 * 2.0
WSD = 842~
Coffinmaker @ 300% TP- WSD = 137 * 3.10 * 2.0
WSD = 848~
The Martial Gun can't exceed the effect of having 300% TP, so anything over 200% TP will be the same. So at 300% TP the Coffinmaker will be slightly better than the Martial...but if you're trying to maximize your DoT, why would you want to hold TP til you hit 300%?! Meanwhile, Slug Shot will be only slightly weaker on the Martial Gun, while equally or possibly more accurate. I have to believe that the Martial Gun is superior in terms of DoT for a COR who melees for TP. If you're shooting for TP however, the Coffinmaker is probably a better choice, with higher D and lower Delay.