Ahh, the memories. AA6OO/Chris, I was in your situation in 1978 when I wanted to get a TS520S as a teenager. Fell in love with the thing, posted a pic from QST in my bedroom/shack corner above my old Johnson Viking Ranger. Took me a long time to save up the newspaper route money and a loan from my Pop. I still have that rig, was just using it this evening on 40 CW.
First, off, I don't have either rig. I have owned a TS520S, TS570D, TS480SAT, Flex 3000, ICOM IC-910, FT817ND. I have also operated LOTS and LOTS of other rigs Yaesu, Icom, Ten Tec, Elecraft, Swan, Johnson, Kenwood, Alinco, etc. and lots of military gear over my 35 years in the hobby and through work.
So my guidance really applies to anyone contemplating the purchase of a rig (and it's not anything official, just one person's perspective, so take it with a grain of salt).
1. Head over to Ham Radio Outlet (Sunnyvale store looks to be about 10 miles from your QTH) or another ham's shack and try these rigs out. It's not a small investment, and you will want to play with each one for ergonomics, options etc.
2. Computer interface...what are your expectations here? Both have capabilities, but not the same. Do you want remote control? Do you want the digital management capabilities?
3. Operations - are you looking to DX, rag chew, do digital, etc.?
4. Have you read the reviews on eham for more anecdotal info? Always good to see what others are saying...
5. Why just the 590 or the 950? What about mobile/portable ops (TS480/FT450). Why not one of the ICOM rigs? (Just wondering, I don't have an ICOM, but I am curious). Have you looked into Elecraft? For a couple hundred more you could get a TS2000 which covers VHF and UHF, would allow you to get onto those bands for FM repeater and weak signal work. You could also get a Flex 3000 in that price range (yes, somebody had to say it, so I did. Please don't anyone "flame" me, I am not a rep for the company nor am I looking for a debate. I simply have a Flex 3000 and I could not be happier, http://wd4elg.blogspot.com
6. Consider buying slightly used, you could save some $$ there also.
7. Save money for accessories: keyer/paddle, software for logging or digital ops, and of course the most important element: the antenna.
Please keep us posted on your quest.