Regarding the encoder on just the RA, vs both dec and RA, I would choose the two encoder solution.
I am not familiar with the iOptron implementation, but I am making an assumption that it uses the encoders to ensure zero backlash. If that's the case, the dec encoder will make a difference in guiding - more so than the RA encoder, since a properly set up mount will not see backlash in RA.
I've never actually understood the utility of having an encoder on the RA axis only. The only practical benefit is to eliminate periodic error. This makes the spec sheet look good, but if you are going to be guiding anyway, you shouldn't generally care about periodic error.
I'd say that ioptron is using the encoder as a means to tame PE. They have a PE spec but if you seen the copies for non-EC versions it can be very smooth or in some cases has a few spikes. If it's in the +/- specs it's still sufficient for ioptron, as least I have never heard of them saying otherwise.
So the encoders are essentially used to give a very flat PE without having to achieve that natively with the mechanical bits. Some would argue that's a bad thing, but then again very expensive fighter jets are designed to be only flyable with the assistance of electronics. And our world is becoming more and more controlled by electronic bits anyhow, so I for one don't really care about this use of encoders.
The other aspects is the the encoders removes the need to do PEC. Which some people feel improves guiding.
My own experience is that the encoders do provide a measurable improvement. So I averaged 0.4-6 on RA and now with encoder min the cem60ec I am down to 0.4-0.5 total with 0.3X being normal for RA. And overall just a better behaved mount.
So in the end it's not that you can't achieve similar with guiding, but I think you'd find it harder and less reliable.
In a nut shell the EC versions are really for people who want a bit better and see still going to accept guiding as a normal thing to do. And it's not too far fetched as eliminating all sources of movement outside of the mount is a serious task and often frustrating.