Looking away from the total capability, just look at PLAN doctrine wise, it looks for Air defense first in its ships and then Anti-ship capability. I wouold expect 054A to eventually be able to fire both HH-16 and HH-7 with a common VLS launcher + 2 type 730 CIWS. You have 3 layers of air defense right there. Even right now, you have 32 HH-16 and 2 type 730 CIWS that gives 054A the ability to do some fleet defense. As I said, if you add the 2 phased array air search radar on top and possibly data-linked with 052C and operate as one complete air defense envelope, that provides fleet support.Interesting comparision between the Aster-15 and the SA-N-12. You are right that the SA-N-12 is normally credited with a greater range and altitude, and both are generally considered to be extremely good, though Aster is probably the more highly regarded - in part because of the natural bias many military observers have to the prestige of the Royal Navy(?). Difficult to say which is the better choice.
Of course, if range is the primary factor then we coulfd edge the Taiwanese Standard armed Perry's ahead in the frigate race but I wouldn't - SA-N-12 is way more modern than the SM-1MR.
Back to the Formidable class, I'd rate its level of stealth as probably much higher than the Type-054a's. Other aspects I'd expect the Formidible to be ahead in are ASW sensors, crew automation and damage control. Probably powerplant also.
But there's no doubt about it, the Type-054a is a very credible frigate which if employed to maximise its strengths, and with proffessionalism, is a very serious threat whoever the opponent is.
I would say from the description of Formidable, it sounds like it will be equipped with more advanced ASW system. The problem is that PLAN doesn't seem to emphasize surface fleet ASW as much as it should. That's probably why China picked Sov, when it had the opportunity to pick bw Sov and Udaloy II.
As for powerplant, it looks like they are both using CODAD and 054A is using more powerful diesel engines.