Go with the Speed-Six.
You'll most l likely never need parts. There are Security-Sixes that have hundreds of thousands of rounds through each, and few need any parts replacements. And if you do, it'll probably be a pawl or something easily obtained.
The older "Six" series are, in my view, much better guns. I don't care for the single-spring design of the GPs, nor the balance. Ruger removed the grip frame from the GPs and then added it (and more) to the barrel and front of the gun, utterly destroying the balance. If you pick up a Security-Six and a S&W 686, you'll find a similar balance (not weight). But pick up either and a GP-100 and you'll find the balance throws the bulk of the weight to the front of the GPs. Like with an improperly balanced knife, one may grow used to it, but as I [gracefully] age, I have a few aches and pains I didn't twenty years ago. And when I pick up a GP, the pain I got in a boating accident years ago shoots down from my shoulder and into the top of my arm and elbow. I think there's a reason revolvers are balanced the way they are, and Ruger destroyed that balance. I'd pay extra and get a 686 if I had to.
Still, it's a personal matter.