I don't have a 30-40 (30Gov't?) but do use some 303Brits.
to me it comes down to brass... IF you can get 30-40 brass, then the 30-40 conversion makes HUGE sense.. even moreso with the AI version
as has been stated above, more case capacity, more velocity with all bullet weights, and handles Heavy's just great... AND that cast friendly long neck! Can be loaded down to equal the '30AI
what the 30-30AI has going for it, Brass... cheap and everywhere. Factory rounds.. cheap and everywhere. can be loaded to moderate velocity in a SB2. Less powder.
in my case, IF I could get my hands on '40 brass, and a gunsmith to rechamber, there would be NO questions
but I can't so, I am sticking to the '30AI
Jeepman makes an offer without peer.... I'd take him up on that IF I had brass.