What's the difference between Natrelle cohesive SRX vs New Mentor Memory Xtra? (Photos)

There is a little bit of mixed terminology here, but in the end the implants are similar.
Allergan's newest line of round, cohesive silicone gel implants are called Natrelle Inspira (brand name). There are 5 profiles (or projections) from low (SRL), low plus (SRLP), SRM (moderate), SRF (full), to SRX (extra-full).
So if you have the SRX implant, it is an extra full profile of the Allergan Natrelle Inspira. Volume is 420cc.
Now, another  company, Mentor, has its own line of new round, cohesive silicone gel implants called Mentor MemoryGel Xtra (brand name).  They are currently available in 2 profiles/projections Moderate Plus and High. 
All of these profiles have a large range of implant sizes (in ccs) from 110 or 200 up to 600 or 800!. 
Getting a particular look in the breasts is much more a function of choosing the correct profile and implant base width and implant volume. While there are some technical differences between the silicone gel and silicone shells in the Natrelle vs MemoryGel, choosing the correct profile/width and volume are very critical.  Speak to your plastic surgeon about it!  Good luck. 

In short, there is very little difference between the two lines of implants.  The Inspira implants with the Responsive gel are very similar to the old Natrelle/Allergan implants, but with a higher fill ratio that should lead to less rippling and perhaps a longer lifespan -- same gel, same shell, better fill (this is what you have, evidenced by the "R" in your SRX).  There are more cohesive Inspira implants, called the SoftTouch and the Cohesive, that have firmer, more cohesive gels from Allergan's shaped implants.  These would be a "SSX" or "SCX" if substituted for your implant.  From my understanding, the Mentor Xtra is simply a standard Mentor responsive gel (same gel as your implant, BTW) with a better fill -- more projection, less rippling.In your case, a more cohesive implant like the Inspira Soft or Cohesive might give you a fuller upper pole that would be closer to your target.

The two profiles of implants are very comparable so you will not see much of a difference there.  However, Natrelle does have more cohesive gels like the Soft Touch and the Cohesive (you appear have the responsive which is the least cohesive) which keep their form better than the responsives and give more upper pole fullness.I would discuss this with your plastic surgeon to get his/her thoughts.Hope that helps.