I was obviously be flippant (hope nobody is offended), but because the SDLv3 is the top of the range eyepiece and the MM3 is the bottom of the range scope.
Nothing wrong with a 50mm scope, but it makes no sense (to me) to have that eyepiece on any MM3. I found very little difference on my MM4 60mm between it and the HDF-T zoom. The only good reason for the SDLv3 (IMO) is if you really need guaranteed waterproofing.
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No probs with flippancy! My scope is the MM3 ED50 angled, not the non-ED 50 which is the current bottom-of-range MM3. I am told that the MM3 ED and MM4 ED are essentially identical optically and mechanically - the only difference is the MM4 has newer lens coatings which give a tadge more contrast, but no better resolution. That said - I am prepared to upgrade the MM3 ED50 body to an MM4 ED50 body, if need be, later on.
As regards image quality difference between HDF-T and SDL v2/v3 eyepieces. Looking at all the forum posts, it seems to me that there are just as many people who think the SDL v2/3 gives a superior image as there are people like you, who think they are much the same. I have seen figures which suggest the SDL eyepieces give slightly wider FoV, if nothing else. Opticron themselves state that the SDL eyepieces give slightly better resolution/contrast - although this may be only a very slight difference! I read that the SDL eyepieces include a field flattener, giving sharper image to the edge than the HDF-T.
The SDLv3 is slightly smaller and less weight than the SDLv2.
Finally, I want the waterproofing of the SDL eyepiece, as I use my MM3 without a case - I have a stay-on-case, but don't tend to use it (I'm not keen on them). So, I consider it worthwhile changing from HDF-T to SDLv3 for the waterproofing alone, in any case. And if, on top of that, I can discern even a slight IQ improvement, then better still.
I am also considering buying an Opticron MM4 ED60 straight scope, as an additional scope to my angled ED50. This is to replace my Nikon Fieldscope EDII 60 straight scope - for use in hides, where I find straight scopes preferable. If I do get the MM4 ED60, I will be able to "share" my SDLv3 eyepiece between the two Opticron scopes, this getting better vfm from it!
That's my logic!
Cheers,
Nick