I had an Ultra and upgraded to the AxeFX II. At firmware v5.0 the AxeFX II was way out ahead of the Ultra IMO. Since then there have been some rather significant improvements and at the current v16.02 it rivals any/all of the some 40+ production, vintage, and boutique amps I have owned over the past 35 years. IMO - if you have the coin to spend then the AxeFX II / XL is a major upgrade from the Standard / Ultra models (and that's saying something as the Std/Ultra is an amazing sounding modeling and effects platform).
It's worth noting that the amp modeling and IR resolution are not the only recipients of Cliff's continued development efforts. The compressor block (studio comp enhanced & 2nd pedal type added), chorus block (dimension type), delay block (2290 type), drive block (all Tube Screamer based circuits), graphic EQ block (fixed and variable Q types, passive types), and the reverb algorithms have improved (from what was an already fantastic reverb) and will be seeing yet another improvement in the next firmware release per a recent post by Cliff.
Oh - there are more than a few new features in the amp block from the Std/Ultra block (major understatement here) but there are a couple worth mentioning IMO.
First - Cliff recently added a 'VariAC' function that allows the user to change the supply (wall) voltage from default (100%) to anywhere from 50% to 150% IIRC (or - well outside safe operating voltage in either direction out here in the real world). This parameter can have a subtle to dramatic effect on the response and character of any amp model.
Second - a bias modulating tremolo effect has been added to the amp block. If you've ever played a pre-1963 Fender Vibrato amp or perhaps an early Valco or Ampeg amp with tremolo this addition will require no explanation. If not then you're in for a treat! (...We'll, assuming you dig tremolo). Very warm and musical sounding effect.
As has already been mentioned cab emulation has taken a huge leap forward between recent enhancements in in the output portion of the amp block and with UltraRes IR's.
Then of course there's the fun in exploring whatever new improvements, parameters, amps, and or features that show up with each new firmware revision. There's no overstating the fun factor here.
Yep - the II is definitely an upgrade.