As far as being 20 goes....Sell the Ruger to your parents to hold onto for your brother for a few years. To buy the P225 from a dealer your parents will have to buy it, then give it to you in a few months.
Get the Sig, but only if you are fine with paying $40 for magazines. If you do get the Sig, buy magazines now while you can. The supply dried up with the tens of thousands of P6s that were imported, for a while last year you had to pay $100 or so for a used magazine.
http://www.topgunsupply.com/sig_p225-9mm-8rd-magazine.html
I have one and love it. The only downside is the price of magazines, which isn't even that bad.
It has been reliable for a few thousand rounds, is very accurate (especially for such a compact gun) and it carries well. It's my ideal "everything" gun because it's compact enough to carry without becoming a burden, but shoots well enough to be used as a sidearm. If I had bought a P225 when I was 20, I'd still have it, which I can't say about any of the goofy purchases that I made at the time.
I got mine for $279 a few months ago. Watch how much you pay for the "P225", there's a good chance it's a police tradein P6. A P225 is more refined, with a better fit and finish and a few different parts, and should cost more than a P6. I wouldn't pay more than $350 for a P6 in good condition (the price has climbed as the supply goes up). If the gun you are looking at is a P6, they're not as pretty as a P225 but they shoot just as well and will last forever if you take care of it.