I had a friend that purchased the Ultra two years ago and he was not very tech-savvy but I only had the AX2 and I remembered they were a little different so I had offered to re-learn it myself for him if he let me borrow it for a weekend (afterwards I could walk him through how to set it up and use it).
I found it was really easy to dial in a good "recording" sound and even a pretty good live sound through my tube power amp with a cab pretty quickly since it was simpler than the AX2 (due to having fewer options and less tweakable parameters).
It helped me to better understand how to dial in the AX2 afterwards.
My friend loved the sound I was able to get with his Ultra through my set-up so he purchased the same things.
I decided to purchase two of them myself since there was a "few" things that I liked better in the Ultra than the AX2.
However, the AX2 still has "more" things that I like better in the AX2 than the Ultra AND there are some things that you could never do in the Ultra that were incorporated into the AX2.
Also, the AX2's received a HUGE firmware update called Ares that really made them come alive.
It can be argued that the effects are much better in the AX2 but many of the effects of the Ultra have that older effect sound that some might prefer over a more modern/cleaner effect sound found in the AX2.
Ultra's are starting to be sold for really cheap these days and if you want to save some extra money then start with an Ultra and learn the Fractal Audio basics while searching for a cheap priced AX2 since they are starting to drop in price as well.
I have seen some Ultra's go for as low as $450 and some AX2's for as low as $650 (it does not happen often but they have gone this low).
I had picked up an Ultra that looked BRAND NEW for even less than $450 with a midi controller (I will never sell it since it was super cheap and I found there were some things that I really liked in the Ultra plus I always regretted selling my Ultra).
You might decide to keep both like me if you can get them cheap enough
The AX8's have been dropping in price as well and they sound very similar to the AX2's plus you get the incorporated footswitches.
I recently purchased an FX8 and an AX8 and I feel they are both keepers as well.
I will always keep my Ultra, XL+, FX8 and AX8, even though I own the AX3 MKII and FM3 since every one of them offer something different and unique
Best wishes!!