The 6.5 is still relatively new to me, but here's what I know.
The 139 Lapua Scenar is an absolute tack-driving dream, but after my poor experience with it on coyotes, I certainly would not try it on deer.
The 143 Eldx has been a bit more finicky in my gun. Of course I'm expecting routine < .5 MOA groups, so I guess "finicky" relative.
Here's what I settled on:
143 Eldx
41.9 gr H4350
Lapua brass
CCI 450 primer
2.220 to ogive
Velocity is in the 2840 range in my 26" barrel. Consistently single digit SD.
I would recommend running a box or two of factory Hornady Match 140 Eldm through it. Maybe not the best deer bullet, but those things shoot amazingly well out of my gun.
Also, Scheels in Fargo (as of Friday anyway) had 8# jugs of H4350 in stock. They have two fewer after my visit.