The actual quote from the movie was "She can still outrun and outgun us".
The "still" could simply be a contextual qualifier. That is to say, even with the repairs Spock was able to oversee while Kirk and party were underground, the Enterprise and Reliant were both still severely damaged vessels, and apparently the Enterprise took the worst damage of the two. Spock's remark could mean that Reliant was not as crippled as Enterprise. This would say nothing about the military appraisals of their respective starship classes.
That is my take and what I tried (however poorly) to convey upthread. That despite Scotty's best efforts to get the ship back up and running at full strength, they were still underpowered in comparison to Reliant. Remember that Reliant had taken a few phaser hits while phaser lock was inoperative. Enterprise, on the other hand, had taken multiple phaser hits, with Reliant's phaser lock operational, not to mention a photon torpedo. She may have fended off Khan's surprise attack, but it was a pyrhic (I just KNOW I spelled that wrong) victory. Yes, Enterprise was okay for now, but Reliant was not destroyed in the counter attack.
When Kirk beams back aboard and tried to use the turbolifts, Spock informs him that they aren't working below C deck (I believe). Kirk asks what is working and Spock replies something to the effect of, "Not much Admiral."
Reliant was in better condition not because it was the better ship to begin with (when Khan first approached Enterprise and initiated the surprise attack), but because of the effects of the attack, for which Scotty was still trying to repair damage.
Just my $0.02.