Wildfowler
Most votes will go to the Swede, understandable considering the UK’s general preference for wider calibres and heavier pills for small deer, with the emphasis on margins of error etc etc (still gotta shoot them in the right place though!).
When it comes to choosing between half a mil of diameter, and (realistically) up to 50gr of bullet weight, you’d think its the bullet weight that’s going to make a difference.
But it doesn’t always work like that, as the choice of bullet in the 6.5 could, if chosen poorly, result in a bullet that is considerably less effective on small animals like roe and yearling red hinds etc, than a different bullet in the .243 Winchester, particularly if the bullet chosen in 6.5 is too hard due to the very high SD they tend to have.
For me little deer like roe don’t need much killing and I’d take to them with a 6mm without a second thought... assuming I have the right bullet, like a 85gr GameKing or 100gr ProHunter. I download my .243 bullets slightly - lower mid range velocity - as I just don’t need them smashing smaller animals at warp speed in order to get the bullet to perform. It’s the warp speed close range shots with soft bullets that give the .243 Win a reputation as a meat spoiler.... and the hard bullets will just fly straight through the animal and away it runs.
In 6.5x55 I’d be careful not to drive it too fast with the heavier hard bullets, which can over penetrate, or the heavier frangible bullets that can be a recipe for big holes on small animals like roe. For smaller deer as you specify, a 120gr soft point in the mid-range velocity is all you’re ever going to need to kill cleanly and maximise recovery. The ProHunter 120gr is outstanding in 6.5mm.
Both cartridges are excellent and I’d pick up a rifle in either and know it will do the job, as long as I can control the bit that flies out the end. Projectile construction, correct velocity and good shot placement are more relevant for regular successful little deer killing and meat recovery than just 0.5mm in calibre and higher bullet weight.
(I disagree with Blobby about Partitions, driven hard I’ve seen them fly straight through light framed, thin skinned deer or antelope, e.g. springbok and impala. For me, they are a .30 cal big deer bullet.)