A 180 grain .308 bullet has a sectional density of 0.271
A 140 grain .264 bullet has a sectional density of 0.287
Yes, a 260 Rem has a slightly higher SD (.016 or 6%), but not enough to offset 40 grains more weight (28%) and 36% greater frontal area in the 308.
In either case, use a premium bullet.
Better yet, for higher sectional denisty are the 156 gr. NORMA bullets - heavy, premium bullets in 3 varieties (see:
http://www.norma.cc/content.asp?Typ=28&Lang=2&Submeny=4&Rubrik=Components )
