Apart from the VERY good points Chris makes, the extra weight,and corresponding handling,AND loss of performance on a MID SIZE RIB [not to mention the considerable extra exspence] coupled with the loss of economy twins represent... would sway me to a GOOD Large Single every time!They are pretty reliable these days!!
Also, and it may be the last thing your thinking about at the moment!! but I think the used market would more often than not, favour a good performing,well balanced, single set up!
Either way I wish you all the best with the new boat!