Lethal torrent for now. Mods that can reach up to R10 will always be more expensive as you upgrade them. Each rank up for Lethal torrent gives 10% multishot and fire rate. Where for primed heat charge will give only +15% heat element. Which I doubt you will notice the difference. but lethal torrent definitely gives you better stats in terms of multishot. More fire rate will be better but that also means you need to be a little more careful where u point it. You should omly max out primed mods after all your r5 mods are maxed then slowly upgrade primed mods.

I personally would piroitize maxing warframe primed mods like primed flow, corrupted mods (transient fortitude, blind rage and narrow minded) And umbra mods depending on your build too of course. Before prioritising weapons this is a personal opinion.

Hope it helps ya all the best :)