RNS-E is indeed much better than RNS-D. RNS-D is useful as a radio and CD player, but now totally obsolete as a SatNav system.
RNS-E comes in two variants: mk1 and mk2. The part number of mk1 versions ends in 192, and the part number of mk2 versions ends in 193. The mk2 (193) versions have a faster processor, a higher resolution screen, support for larger SD cards, enhanced software, etc, etc.
The mk2 (193) versions can then be further subdivided into variants B, C, D, G, etc. This determines, first and foremost, the shape of their fascia. Different Audis have differently shaped centre consoles, so the plastic fascia of the RNS-E units needs to vary. The letter on the end also determines what auxiliary functionality is included. For instance, the G versions do not have the necessary internal hardware to support a voltage-triggered rear-view camera or an analogue speed pulse. Although aftermarket solutions exists to overcome this problem, the higher-spec B versions have all the necessary internal hardware to allow seamless integration with the A2's electrics.
In short, if you're looking for the best Audi SatNav system for the A2, it's the 193B.
Cheers,
Tom