Which fights would that be? I'd rather use a DG on-use for every fight in the new content (EC NiM + TFB HM), sure there are some like the last boss in TFB where you have to stop a lot, but the enrage is ridiculously easy there and I'd rather have the cooldown for the last 15%. Having the on-use burst and the overall higher damage is much better.
EV: XRR-3 (soft enrage at end - but it really isnt necessary with current gear levels), Soa (questionable, you can only use the power relic on one, maybe two of his shield break phases so sustained on 4 is probably better than popping it on 1).
KP: Gv-B3 Heavy fab (you can prob hit it on every other burn, depending on how your raid is doing this fight since it can be accomplished in multiple ways)
EC: Tanks (I am only putting this on here for Nightmare, since if you can time with inspirations [if you have 2+], it would probably be better -- for any other mode it is unlikely an on-use is better), Kephess (purely for the warstrider and the last burns on kephess)
TFB: Kephess (use here is obvious), TFB (there is a large enough downtime between certain phases and enough static points where on-use is probably better than a sustained - definitely on the last 15% burn)
I'll also note that I was speaking in general - best case scenario is you have your WH, your on-use, and your proc and you can use them in each scenario that is best suited. With only 4 or so fights out of 19 better with on-use, I'd prob go for sustained.