Yes, it is a good trade.
Go look at the prices of early Ruger 9mm revolvers. I know for a fact that SP101s sell for around 900 dollars, and I've seen them peak around 1250, mint with box and all accessories.
The 625 is a production gun, many many more of them made than the Speed Six. I would jump all over the Speed Six if I were you.
If you don't get it, can you send me a link? I'm interested, it'll pair nicely with little brother SP101 in 9mm that I have.