Need more specifics to give a better answer.
If the buying price was the same and both engines in about the same condition, buy the 261.
More cubes more horsepower.
235 and 261 share a lot of common parts but 261 specific parts are more expensive.
See the Tech Tips section for Engine Swap (216 to 235/261).

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."