IDK where you are getting your numbers but you clearly missed that Gromp buff also deals 5% of your bonus hp over 3 seconds. Small camps also do about 50% of a large camps damage (varies by camp) so killing them early actually saves you a lot of health. For reference Red Brambleback does 80 dmg per hit. The 2 cinderlings do a combined 24 damage per hit at the same attack rate but are all but impossible to kite due to their range. As such with proper kiting mechanics they do attack almost twice as frequently as the Brambleback, doing 48ish damage for every 80 damage Brambleback does. Killing them earlier effectively cuts that camp's damage by about 1/3 .
If you get the stun on Brambleback 2 times while clearing it you reduce the damage you take by 160, not 300. If the cinderlings deal 24 damage every .6 seconds then if you kill them 11 seconds sooner you negate that damage.
At level 3ish Sejuani has about 800 HP w/o runes/masteries. With runes and masteries she should have about 100 bonus health (50 from veterans scars + 30 from strength of ages + 20ish runes) That means Gromp buff is doing about 10 + 5 damage over 3 seconds or 5 damage a second to all 3 of them which means that in the 10ish seconds it takes a level 3 Sej to fully clear Red Buff (she is pretty slow about clearing early) it will have done a total of 150 damage spread among the 3 monsters.
It is also procing the Hunter's Talisman's passive Tooth dealing 50 additional damage to each of the cinderlings and restoring 100 health that a Sej wouldn't have had with proper kiting (A sej who doesn't kite would have gotten the healing but would have taken much more Bramblback damage) This takes the base 160 damage that Heavy hands would have negated, and reduces it by 100 to break even. This leaves you at 60 damage that needs to be negated by the faster killing of the cinderlings. Over the 8ish seconds it would take Sej to kill Red brambleback they would have taken 80 damage each, saving Sej 2 AA's worth of time vs each. Sej has almost the same AS as the cinderlings at this point. So in 4 AA's time (with one cinderling dying 2 AA's in) She would normally take 72 Damage from Cinderlings which has now been negated. This means that with excellent kiting Gromp buff is marginally more useful on Sej than Gift of Heavy Hands.
Overall the difference is minimal and changes as you clear more camps plus the thousand and one different variables that have to be taken into account. Basically they are the same in terms of efficient clearing on someone as slow as Sejuani.
If you don't have good kiting then Krugs is definitely better so long as you are using your aoe to hit the small camps as well as the big ones.
TLDR
They are basically equal on champs with similar attack speed to Sej who take Strength of Ages.
(maxes E in the jungle with AS runes) 
(if you don't have to start E, you have an extremely healthy clear; you do anyway if you have to start gromp side) 
all essentially do not take damage, or as significant amount enough to matter, and can use the extra clearspeed+sustain (it trades HP from the monster to you, not just healing you) via tooth+
.
(which now gives hp / level) 
you will force an 
meta into play where the red side just picks a strong early adc + 
and forces you to either bite the bullet and get fucked as blue side or swap lanes and lose turret pressure due to upcoming game dynamic changes meant to nerf lane swapping.
