I'm not sure what the initial publication / upload date is for this info but it's been around quite a while now. Dating can be approximated by reference to Winchester offering 25-06 Remington factory ammunition loaded with the 110 grain Nosler Accubond at published 3100 fps MV. Winchester no longer offers this load, but Federal has for a while, including 3100 fps published muzzle velocity.
https://www.ballisticstudies.com/Knowledgebase/.25-06+Remington.html
Excerpt on general performance
"Debate is always rife regarding the game weight limitations of the .25-06. The .25-06 is often declared a lighter medium game cartridge but realistically the .25-06 would fit the needs of most hunters world wide. This is an ideal cartridge for a great many of the worlds deer species so perhaps it is best regarded as simply either a medium game or deer cartridge rather than a lighter medium game cartridge. On light medium game, the .25-06 is a spectacular killer to say the least. On larger medium game, the .25-06 can be relied on to produce clean kills. This cartridge is really at its limit on Elk sized game for ordinary hunting / cross body shots if fast humane kills are to be expected. The .25-06 will indeed handle much larger animals with carefully placed head and neck shots but such situations should be considered the extreme."
Excerpt on the Nosler Accubond factory load previously mentioned.
"For all-round use, Winchester’s 110 grain Nosler Accubond load combines high BC’s and explosive performance with optimum penetration. The Accubond gives deep penetration on light bodied animals but will not penetrate vitals from tail on shots on larger bodied deer. Winchester suggest that the Accubond is best suited to light thin skinned game however the 110 grain Accubond optimizes the performance of the .25-06 for those who wish to hunt game the size of Elk (320kg/700lb) which are truly the upper limit for this versatile cartridge."
Current Federal factory ammunition loaded with the 110 grain Accubond with published muzzle velocity of 3100 fps. I've had excellent performance from this load on a feral hog.
https://www.federalpremium.com/rifle/premium-centerfire-rifle/nosler-accubond/11-P2506A1.html
Excerpt on two other Federal factory 25-06 Remington offerings.
"Federal’s other offerings are the 117 grain Sierra GameKing and the 115 grain Nosler Partition at 2990fps for 2920fps in shorter barrel lengths. The Sierra is an outstanding longer range deer bullet, again, with suitable shot placement. This projectile opens to a very wide diameter, will almost always produce an exit wound on typically lean bodied animals and produces fast kills. Likewise, the 115 grain Partition is an extremely reliable projectile. To some extent, the Partition does away with the need for the GameKing load. The Partition opens quickly and produces just as wide a wound as the GameKing but also produces far superior penetration - ideal for wild boar as well as woods hunting or deer at awkward angles. The 115 grain Partition is capable of giving full body length penetration on lighter species of Whitetail deer."
Underwood offers factory 25-06 Remington ammunition loaded with the 110 grain Accubond with a published MV of 3250 fps, so does (did?) Doubletap, both have had inventory depleted during the pandemic pandemonium, and I've personally had a far superior customer experience with Underwood than Doubletap.
https://www.underwoodammo.com/colle...osler-accubond-spitzer?variant=18785712242745
Doubletap includes an elevation of 5300 ft in their calculated performance data
https://doubletapammo.com/collections/all/products/25-06-remington-110gr-nosler-accubond-20rds
Here's a comparable writeup on the 6.5X55mm
https://www.ballisticstudies.com/Knowledgebase/6.5x55.html
Excerpt on the 140 grain Nosler Partition
"
The 140 grain Partition is without a doubt the most effective all around projectile for the Swede. Nothing else comes remotely close. This is a bullet that time after time, produces a deep, broad, violent wound resulting in fast kills. The Partition exits medium game at speeds fast enough to ensure complete disruption of vital organ pressures. Light and lean or large and tough, this is the go to bullet in the Swede, reaching its limit (wide wounding) on game weighing around 150kg (330lb) although it is adequate for use on Elk - unless you are sold on the idea that 450kg (1000lb) body weights are what the Swede was designed for. Wounding is wide down to velocities as low as 2200fps (310 yards) becoming moderate as velocities approach 2000fps or 430 yards. For those who use the Swede and have not hunted with this bullet, try it. The Partition should be driven into the major bones of the forwards chest cavity on game of all weights, not because of the bullet design but due to the power limitations of the Swede. Used this way, the Swede is brought to optimum performance."
Doubletap also offers 6.5X55mm ammunition
https://doubletapammo.com/collections/all/6-5x55-swedish
We live in an era with a plethora of both variety of cartridges and projectiles. Now there's Hornady ELD-X, Norma Bondstrike (in 6.5mm), Berger bullets, Federal Fusion, and so on that aren'tpart of these articles.
If one wants to compare theoretical performance differences there's a Ron Spomer article about 25-06 Remington vs 6.5 Creedmoor, could substitute 6.5X55mm for 6.5 Creedmoor, FWIW. There's plenty of history of 6.5X55mm performance on game available for review.
https://www.ronspomeroutdoors.com/blog/25-06-remington-beats-6-5-creedmoor/
OP, I'd choose more on what's your long term plan for whichever rifle you'll end up keeping. Family keepsake? Game and field conditions? Name prestige? Personal "fit" for you or your descendants?
Which provides the greatest long term value for you, however you define value, is the one to keep.