Both great bikes
I want to stress both are absolutely outstanding bikes (I had an X5 for a short while, but felt it handled a bit slow for me, so I went to a salty).
Differences - in no particular order and in no particular importance.
Handling - 5.5 is much more XC, particularly now that the X5 has a more slack HT angle than the original X5. I wouldn't say the 5.5 is quick but it's neutral and works great for me in singletrack etc. Also, I ride mine at 125-130mm up front in most situations. I will use more travel (getting a new talas soon) to slacken it out in steeper situations.
Suspension - 5.5 is more effeicient pedaling, however platform shocks have neutralized that difference a bit. The 5.5 does however accelerate quicker and has less bob. The 5.5 does have a bit of pedal kick-back in the granny gear. The x5 is a bit more neutral in all chainrings. X5 will be more affected by shock selection (coil vs air), 5.5 is a bit more particular to shock set-up (as opposed to selection).
Build quality - Ventana is a 10 out of 10, Intense 8.5 out of 10. Both excellent. Subtle differences.
Stiffness at the bb and rear end. Ventana 10 out of 10 (with double bearings) Intense 9 out of 10 (I have the new upper link). I'm 205 lbs or so. I do occasionally get a SLIGHT rubbing on my rear disc brake on the Intense in real tight switchbacks. We are talking slight, and not a deal breaker.
Customer Service - No one is better than Ventana. Have not had a reason to use Intense yet, though I've called in the past and Ventana is a bit more "personal" with you on the phone.
Honestly, if you are looking at these 2 bikes, handling might be the biggest difference, followed closely by suspension design and properties. I think handling wise you would be better off comparing the 6.6 to the X5 and a Salty (even with 5 inch rockers) to the 5.5.
Both great, great bikes.