I have both. I saw a thread where someone pointed out the VBass factory patch is on both synths, so I've been using that as a comparison. I don't know if it's all in my mind but to me they sound slightly different. The AS-1 filter does step just like the P6, but I swear it seems to be ever so slightly smoother than the P6. Nothing that would make a difference in a track, but something I could both hear and see visually when using ProQ3 plugin to watch.. I made sure they were tuned the same and levels were same during my comparison. Now that VBass patch does have a bit of movement in it so playing it on two synths side by side they tend to sound different just because of the phase isnt in sync on both. But overall I feel there is some sort of extra bass presence going on in the AS-1 -- perhaps difference in gain staging? I don't mean like low-end in the EQ sense, I just mean the overall presence, like the filter is a little stronger or something. I did notice the AS-1 seems to put out less noise, which might contribute to the bass sounding a bit better (I'm just not sure if that has something to do with my particular setup).... So overall that makes the AS-1 it a slightly (and I do mean very slightly) superior bass synth when comparing sounds in the typical "synth bass" range of tones. I feel like the difference I'm hearing is similar to what others have said about vintage Pro-One versus the P5 back in the day.
Of course the P6 can do way more than just great basslines, so for me the real bang for the buck here is that I can pretty much relegate most of my bass duties to the AS-1, freeing up the P6 to simulatneously perform many other roles. But the fact that the P6 can hang with the AS-1 on bass, plus hang pretty well with the P10 on pads, not to mention do many things that none of the others can do, just shows what a beast the P6 really is... a truly versatile beast.
The AS-1 I will highly recommend, especially for those wanting a killer bass machine. Great for mono leads, analog drums and so forth, too... its just that in my own studio it will be mostly only used as my go-to bass favorite.