3U was my most played MH but in my personal opinion, 4U is better than it in almost every way. The monster choices are potentially questionable in some aspects - no Duramboros, Agnaktor, or some of my other favorites from 3U, and bringing back the drones over the ggis (except Great Jaggi is still in the game). But with that there is a much larger selection of monsters and a lot of really damn cool ones which weren't featued in 3U.
There's also some other changes made
So I dunno, it's up to you. 3U was a solid title but 4U is practically objectively better. Just depends whether or not you want to finish 3U before you move on to it.
Also how long do you reckon I will be spending on MH3U with my current pace.
Quite a good amount of time. I think I have maybe 100-110 hours on 3U (well at least the 3DS version) and I was on the 9* quests with almost every mission completed. So if you're just hitting HR at 80 hours... you got a ways to go. My best tip for high rank: get high rank armor right off the bat. Pretty much the worst high rank armor is still more defense than the best low rank armor, you're going to need it.
In my personal opinion, I'd say just hop into 4U. You will sink tons and tons of time into it and it's fairly difficult in comparison to 3U so starting back at 4U low rank might be a bit of a step up in challenge for you anyways. I know in 3U I pretty much never fainted on a low rank mission, but in 4U I fainted on quite a few monsters in low rank. You don't start with the base 50 armor that 3U gives you so they hit harder. I'm pretty much freshly into high rank on 4U and I'm like 55 hours in.