Gonna try the polaris, it has a bit less ammo than what I want but I never used an LMG with a laser scope, so who knows! After ~9200 mcg kills and ~4500 with the lasher (I just discovered how incredible this weapon is when used hipfiring) I think my gamestyle is much related to good hipfiring lmgs rather than raw high dps ones.
Wait, you say you discovered the Lashers hipfire just now? Back when, it was more accurate than todays ADS
Rhino and em1 are comparable to the polaris instead, which is a nice gun. But out of your original three I would pick ursa, very low and controllable recoil makes it easy to hit headshots at any range.
Personally I'd just use the NS-15m, since it's basically a more versatile and all-around better version of all 3.
The Ursa has slightly better DPS then the NS-15 and less side to side recoil. The only bad thing about it is the low RoF makes it a little harder to hit moving targets at range. Up close the .75 ADS speed, higher rof and better hipfire make the NS15 superior. Then again a high rof 50 round mag on a 143-125 dmg model is awful for break down the doors room clearing. If that is the kind of game play you want, then for that a 150 round Flare with SPA and Nanoweave/Adrenaline Shield is just OP. If you want to spam at targets down range, a Lasher is ideal. Ursa if you want accurate down range fire. NS15 if you want to shoot at targets far away and up close but prefer hit and run to Rambo play style.
Personally, I think of Polaris/Rhino/Em1 to be like auto scout rifles for a heavy. Lower damage, but the ads accuracy lets you get kills from headshots. I've never really used them for hipfire, feels way too awkward for me. More like burst headshot rifle. Off the top of my head, my favorite hipfire lmg for cqc is the flare, at only 250 certs. It has a fantastic recoil pattern that can be used to your advantage. Because of its recoil pattern I actually prefer it at closer ranges and hate it at long ones. Still wouldn't ever bother with a laser sight, and I do like to ads at 10 meters out to medium range. It has a 75 round capacity that can be extended to 150.
