IMHO being in like condition, they both hold their value about equally. That being said, the model 66 2.5 inch will always sell for more money, and the 3 inch will sell for the most money. Just my 2 cents.
Ditto...... the 2 1/2 inch would sell for $100-150 more around here...... the 3" 66s are generally in the $1000-1400 range