DX11 is going to be better, simply because it's more useful than DX10 ever was. It does look better, but the real treasure is being able to make things look better without hogging up GPU resources. At $100, the best Dx11 card is the HD5670, but if you can put out $40 more for an HD5770, you can get twice the performance.
The new DX11 games that are coming out now have a DX10 or maybe a DX9 mode which will allow you to play with older DX10 and 9 cards, but don't expect programmers to allow that backwards compatibility forever.
I agree with shadow about the 5670 and if you can spare more a 5770.
DX11 doesn't really mean much at all "today" as the games released to date have only implemented it to a minor extent. If you read the dirt 2 article for example here on THG, it labels the impact on the gaming experience as barely noticeable but the impact on performance as "colossal".
DX11 allows game developers to make more detailed by using a greater number of polygons. As it takes about 2 years and as much 20 - 100 million dollars to develop a game, and DX11 only being around for 4 months, devs have hardly had time to heavily implement its features.
If you are looking at two cards that perform about the same and one is DX11, by all means grab the DX11 card. But if the DX10 card has a performance advantage, I'd lean toward the faster card. The key will be how long you expect to use the card....if you will get a new GFX card again within the next 18 months, I don't see not having DX11 being a big disappointment.
You don't mention resolution which is the key factor in choosing a card. here's what THG says:
Best PCI Express (PCIe) Card For ~$80: - GeForce 9600 GT - Great 1680x1050 performance in most games, 1920x1200 in most games with lowered detail
The February Roundup has me a bit confused as in January, THG had a tie between the 4850 and the 250 at $110. This month the 4850 sits alone at the $100 spot which I didn't understand as until yesterday, they were selling for the exact same $100 price on newegg. Which one to buy ? Quick answer, whichever one is cheaper......tho, if ya think ya might wanna play any PhysX games, you could still find some use for the 250 in your next build as a dedicated PhysX card and the extra $10 you'd pay at newegg today might be worth it in the long run.
If you think you'll live with this card for more than 18 months, ya might wanna move up $40 to the 5750.
Over the long run though, I'd urge you to move up to something more powerful, as a 1920 x 1080 monitor, which are now at the $200 price point, can't be far behind. A 5770 will do DX9/10 very well and you can always add a 2nd one when DX11 games start to mature and place more demands on GFC systems.