For a minute it looked like Stamkos was going to take the goal scoring crown from Ovie, but then he wrecked his leg and O came roaring back.
Stamkos had a chance early but the real separation came with Ovechkin's uncanny ability to sustain his goal scoring through his late 30s.
Ovechkin vs Stamkos (note that I'm using their 'age' season for these comparisons)
18 - 24 Comparison
Ovechkin had 269 goals (only 5 seasons)
Stamkos had 276 goals (7 seasons though one was prorated due to the 12-13 lockout)
25 - 29 Comparison
Ovechkin had 206 goals (one prorated due to 12-13 lockout)
Stamkos had 146 goals (lost most of 16-17 to injury)
30 - 34 Comparison (Stamkos is in his age 34 season; this is where Ovechkin blows Stamkos away)
Ovechkin had 231 goals
Stamkos has 151 (with about 20 games left in 24-25)
Note that Ovechkin has 178 goals in his 35-39 age seasons with about 20 games left in 24-25.
If 24-25 is a blip, Stamkos has a reasonable outside shot at 700. If it's not a blip, he'll probably land around 630 (assuming ~20 goals per season). Stammer is on pace for 24 goals this year with a reasonable shooting percentage (13.7%).
Bonus fun fact: If you give Ovechkin no credit for his three seasons prior to Stamkos entering the NHL, Ovechkin still leads 721 to 573.