True, but I believe the rng to spawn the gobs happens at rift creation. So you could be abandoning a rift before you meet the gob pack. I've been in many pub games where someone had hit a gob pack after the rift had been closed and timer is counting down. Also, I will always complete a rift that has a gob pack at the front, as I'll often run into another set before the end. Anecdotal I know, but it has been true for me.
If you're just hunting DB's then you should likely clear to the end of the rift regardless as it gives greater chance to run into gob packs as well. Previous seasons I would definitely say impale is the fastest way to to just farm but with how strong multi-shot is now, it may be just as fast since you will one shot trash. Either one will be really nice for this now. I like to pair a 50 hoarder gem, averice band in cube, warzche.. armguards with multishot for maximum movement. I will also sometimes max out my hatred in paragon and pick up blood vengeance passive just to ensure I'm not waiting on hatred while speed farming. Discipline can sometimes be the issue for me on multishot but not as much with the hoader, averice, w.armguards combo. I would usually do the multishot build for T10 if I wasn't strong enough and use impale for pure DB farming at T13. Now it is going to be OK to farm at T13 with UE right away due to how strong the build is. Sorry if I have not helped.
TL;DR - Either one will be strong enough now for T13 farming. Don't worry about closing the rift, just keep farming to the end (unless you are keystone starved).