At this date, the 270X is on par with the 660 Ti. They are about the same price however the TI has a superior packaging (unless you value Battlefield more than Splinter Cell Blacklist AND AC 4) The Ti is $10 cheaper and has about 3 frames lower than the 270X. Granted both cards max out Skyrim at 60+ fps.
What is comes down to is Do you like Nvidia or AMD more?
A few notes though:
If you will use a bigger resolution I'd go for the 270X (but the Ti produces slightly slower results)
http://www.hwcompare.com/15672/geforce-gtx-660-ti-vs-radeon-r9-270x/
the Ti has a lower power consumption by 30 watts.
Also the cards will trade blows in benchmarks as they are on par.
http://www.techpowerup.com/reviews/AMD/R9_270X/13.html
This page shows it.
Also the 270x can substitute PhysX however the CUDA core by Nvidia allows for Advanced PhysX with special exclusive things that these videos show:
http://www.youtube.com/watch?v=H4ACKAUU3O0
That is Water.
http://www.youtube.com/watch?v=8ztQX0C4wTE
This is a destructive world Demo (please note you can break apart the broken particles indefinitely.
Actually the 270X cannot do GPU PhysX and instead has to put it to the CPU for the particles. (which is generally slower)