Either is going to work, and, the guys that designed these combinations have had guys go to Africa, etc. and prove they work.
Both look like excellent, well designed rounds.
Pick which ever one you like, and go with it.
I like Tim Sundles, and Buffalobore. Don't know much about Garrett other then his stuff has been used in Africa, and, since he only makes .44 and 45/70, I would guess he's done most of his R and D with those two.
I've ordered from Buffalobore many times.
Garrett has some good stuff on his website:
http://www.garrettcartridges.com/defensive.html
http://www.garrettcartridges.com/penetration.html
Pretty obvious there is more to hunting big game then just penetration.
Also, it's pretty clear he has one goal: Selling his 500 grain Hornady 45/70 load is this article.
A good place to double check Garrett's observations are here:
http://www.handloads.com/misc/linebaugh.penetration.tests.asp
What I can say is recoil is going to be huge:
Out of an FA 83: 330's at 1400 fps.
Recoil Energy of 28 foot pounds, and Recoil Velocity of 24 fps.
Keep in mind that you are pushing the limit of what the .44 can do, so you are going to pay the maximum load recoil penalty, which is when you get up around maximum, or more loads, you get a geometric increase in recoil, for a
sum increase in velocity. NOT a good trade off.