I removed 255's and went with the 259's along with a "Race Kit Stage IV" 103". Prior to that the 255's With intake, exhaust and a dyno tune got me 92 hp and 109 tq. The bike felt really good, power down low is as good as it gets, period. Revs to 4500rpms with tons of pulling power from 2Kup. It was very good for a bagger.
I wanted more power, a 120R was where I was going when the HD folks called me with this kind of hard to find Race Kit 103 that would pull 110/110 numbers. It was nearly 2K less money and left me the 120R for a future power increase. The numbers were close to advertised, with 117hp/111tq my gains were mostly in the top end area as compared to the 255's. But compared to stock, I am light years ahead from 2500rpms on. It has more bottom end than stock, almost up to the 255 level until 3Krpms, then the 259's pull away fast and the hp is still climbing at a still pretty steep angle when 6250 redline hits. I wish it would wind to 6500, I think it would break the 120 hp easily.
As a guy who has had both of these cams that the OP has asked about, I would say what type of riding and what bike. The 255's for the touring bike with load, long distance and no need to down shift is what they do best. The 259's are great for lighter bikes, my RGC, would be as heavy of a bike as I would use them on. For hotrodding and the like, 259's are the way to go.
103" w/Race Kit,Stage IV 117HP 111TQ
SE 259E Cams
SE Heavy Breather AC (Scott's modified)
SE Race Tuner
V&H Power Duals & Hi Outputs
SE CNC Heads 10.62/1 w/SE 58mm TB
John Scott port matching
Renegade Racine wheels. 21x3.5/ 17x6.25