Honestly the calcium ascorbate has worked fine for me, but on the expensive side.
The concern is about it dragging calcium into tissues it does not belong and interfering with cellular metabolisim, but honestly all buffered forms have a lower activity rate (I've tried sodium ascorbate and calcium ascorbate to date).
I still swear by ascorbic acid, the effect is immediate. If you blend a tiny bit of ALA into the powder, it's got a good kick. (My current mix is 800g ascorbic acid to 1200mg ALA)