I've posted this opinion (here & elsewhere) a number of times when others have asked the same or similar questions. I guess I'll offer it again...
With the Ultra, you have extra effects and tools that don't exist in the standard. The Ultra has an arpeggiator, synth effects, vocoder, looper, multi-band compressor, ring modulator, quad chorus, diffuser, resonator, crossover and more complex delay and pitch based effects. There's now also that cool perceptual reverb thing that Cliff shared with us. These don't exist in the standard. The Ultra also has more memory and a faster processor. This allows you to have more elaborate patches and presets. You can run multiple amp models and cabinet models at the same time with more effects. It allows you to be more creative. Both AXE FX units allow you to route all amps, cabinets, and effects in any order or arrangement that you can think of. The Ultra's added power and speed allow you to be even more creative than you already can with the standard model.
When I made the purchase, my thought process was this... If I am going to spend this amount of money, I'm going to spend a little more and do this one time (the correct way). You will be happy with either one. But, if you have a high musical or guitar skill level and some experience, you should get the Ultra if you can afford it. Again, both are great, but there are many extras with the Ultra.
Also, I have read the comments of many people in the last year who own (or owned) a standard and say that they either wish they had an Ultra, or tell us that they bought an Ultra, eventually. Conversely, I have NEVER heard anyone who owns an Ultra say that they wish that they had saved a few hundred bucks and just gotten the standard.