First off if I was mimic it would depend on my powerset...
Like if I could pick:
Mr immortal
Northstar
Blink
Magneto
Colossus...
__________________ Quotes from Hia8:
"I claimed that the science is sometimes faulty."
"You don't understand. This is fiction. That means none of this stuff really happened."
"There is no writer to purposely ignore a character's natural ability just because it suits the story."
"in some cases because the writer knows that Character A will dominate Character B easily and refuses to allow this to happen for the sake of the story."
I'd pick Superboy. I'd become a professional athlete with an extended career, accumulate vast wealth (from playing and endorsements), and spend the subsequent centuries creating a financial empire to provide for my family and friends through the years (altering my appearance and identity so as to not raise any suspicions about why I am so long lived), and I'd commit thefts of gold, cash, etc overseas to subsidize my earnings (only if necessary).
I'd also spend a lot of time in Brazil sun bathing and having my way with the women as I'd be immune to any and all Earth diseases.
My choice of Superboy's abilities is based on his being the least conspicuous. As Thunderstrike I have to carry around a mace, and as Mimic there's the becoming metallic via Colossus which is far to obvious. I'd prefer keeping my abilities under wraps.
If we're using Kon-El/Connor, I'd say with regular sunlight he can get at least a few millenia of life under his belt if not more considering he was genetically engineered to not age and his progenitor Kal El is pretty much immortal (Connor's template being based on Kal's Kryptonian genes).
Using the Kal El Superboy, on the other hand, I'd be immortal.