it depends on preference and budget of course
do you want the whole system new or would you be willing to go second hand ?
you can pick up a single m&p 3in1 on whatever chassis u decide really cheap second hand and brand new(or nearly new) and most ppl sell their duette chassis seperately, although every now and again you might get a full set up on duette chassis, with/out a single chassis including
but whether u buy a single or duette chassis you can get them so cheap on ebay, and choose swivel wheels/classic
i bought my 3 in 1 for dc3 due in dec off ebay lately and my mum say it and thought it was new i bought on a classic chassis but will be picking up an mpx(swivel wheel) for about £60 just so i can switch between as i like classic for long walks but swivel wheels for shopping trips,
have to say my 2 dd's are 7 and 5 now and i had the duette for them 5 yrs ago and im still raving about it, i think it was the best thing ever invented dont know why m&p discontinued the duette chassis, but type in mamas and papas on ebay and it should bring one up not sure whats on at mo, just so you can an idea of the set up as its quite hard to explain.
my main reason for choosing were i wanted something that could carry 2 children but 1 whne needed, and i wanted something i could use with a carrycot(there was nothing on the market at the time, that had that option)i also wanted something i could put a car seat,and a seta unit on i also needed something that would provide a full recline position for older dc(which was also something you dont get from most tandems) i also needed something that could let the lightest(youngest) ride at the front as i cant lift a heavy toddler at the front (like most tandems) due to an injury
you would be hard pushed to find something that fitted my requirements now never mind 5 yrs ago, when jane twin and phil and teds i dont think even existed.
sorry for long posted once i egt going about mamas and papas i cant stop just love em how sad