That's the part only you can answer - what's best for the long run for YOU.
My personal experience - and only mine so not meant to refute anyone else's...
Sony DSLRs are fairly readily available in my area, so handling them and trying them out isn't a problem. I haven't had any support issues with Sony cameras personally. I find the quality lenses with a Sony badge on them can be pricey, but the quality lenses on the used market are actually a bit cheaper than other brands' equivalents, especially considering whatever lens you get is stabilized.
As for 'support' from Sony on the DSLR front - Sony has released 9 APS-C cameras in just 1 year, with 2 more clearly in the pipeline, and another fullframe rumored soon too. They've just firmwared their NEX and their A850/900, which seems to hint at continued support. When it comes to professional-grade support...that's something Canon and Nikon can offer for their pro bodies, and Sony cannot - so if you're going pro and full frame, stick with those two. But remember that Canon's entry and mid-models don't get that same pro level support, and a large majority of shooters aren't professional photogs who will need a replacement $25,000 lens flown into Tanzania by helicopter same day. For regular customer service, repairs, etc...I've personally had very good experience with Sony on the only camera I've ever had to send in for repair (F717 to replace sensor under warranty recall, after 4 years). I can't really say if they've gone downhill since, as I've never experienced any other failure or repair need with 9 Sony cameras over 13 years.
I had no existing lens collection, but I saw a clear availability of the lenses I wanted to expand to within the Sony mount, and cheaply, which is what made me decide on them. I was able to pick up a good travel zoom (18-250) for $750 - stabilized, a megazoom wildlife lens for $700 (Tamron 200-500) - stabilized of course, which isn't available for this lens in other mounts. I got a 50mm F1.7 for $50 - stabilized. Picked up a 30mm F1.4 for $400, stabilized. Got a 10-24mm UWA for $450, stabilized. Picked up a solid Tamron 90mm F2.8 macro for under $100, stabilized. My beauty lens I've lusted after, the Minolta 300mm F4 G APO, I picked up for $900, stabilized. So without an existing lens collection, Sony still offered a solid path to upgrades for little money.
Since my first Sony body, which was a great performer for good light, but not great in low light, and was a bit slow for burst mode and lighter on dedicated controls, I then upgraded to an excellent high ISO performer with very fast AF and burst and more dedicated controls...and the newest DSLRs recently introduced have upped the spec even more over my current body, making the upgrade path in bodies look pretty good for me.
Personally, I don't really understand the comment about fitting Sigma and Tamron lenses to a Sony body 'defeating the purpose'. If the purpose is to get good lenses at good prices to get good images, I find Sigma and Tamron, and Tokina too, have some very nice lenses well worth the purchase, no matter what camera brand you are using. And I enjoy that all are stabilized to boot on Sony mount. I've got good lenses from Sony, Minolta, Tamron, and Sigma in my collection, and see no reason that any of them are better or worse than any other. I've seen some junky Tamron and Sigma lenses - but I've also seen junky Canon, Nikon, Sony, Minolta, and Pentax lenses. Everyone makes some cheap lenses and expensive lenses, cheaply built lenses and solid tanklike lenses. Sigma and Tamron are no exceptions. Again, just my opinion.
If you are not going to feel happy, or secure, with a particular brand, avoid it. It's that simple. Some people will only feel good if they buy the best-selling brand - if that's you, head to Canon and be happy - they make fine cameras. Otherwise, check out all available brands, hopefully in person and in-hand if you can, and figure out which one seems like it will work best for you. You won't go wrong - they all make good products and have fine lenses available for them.
Sony DSLR-A580 / Sony 18-250mm / Minolta 50mm F1.7 / Sigma 30mm F1.4 / Tamron 10-24mm / Tamron 150-600mm / Tamron 90mm F2.8 macro / Minolta 300mm F4 APO
Sony A6000 / 18-55mm F3.5-5.6 / 55-210mm F4-6.3 / 10-18mm F4 / 35mm F1.8 / 16mm F2.8 / via manual adapter, lots of Pentax K mount, Konica K/AR mount, and Leica M mount manual lenses