Hi, and congratulations on your twins. I was in your position a year ago, have a 20 month gap between my dd and twins.
I got a valco zee two with removable toddler seat brought over from the USA, (one of the only positives to my DH having to do 2week stints there 3 times a year). It has honestly saved my life. There is no way on earth dd would have walked at just 20 months, she still likes to be in the buggy even now at nearly 2.5. It depends on your lifestyle. We live a 15 min walk (at my speed) into town. If I drove then dd would be perfectly capable of walking round town, but she couldn't walk there and back as well, and I prefer to get a bit of exercise when I can rather than driving everywhere.
I've not tried one, but if you look at www.lotsofbabies.com (I think) they sell the t3 jogger which has a removable toddler seat. They do come up on eBay fairly regularly. Had I not had the opportunity to get a valco brought over then I would have had one of these. My buggy is much lighter than this one, but of course is not tested to uk standards (but it's tested to US/Aussie ones so I'm confident it is safe)
My reasoning for a triple was that with the age gap I had, there's no way I would have done without a double if dc2 had had the decency to be a singleton. Her needs didn't change just because I had twins, therefore I went our the triple. The thing with the sling option is that it just isn't practical for a longer trip out. Plus think about how long it would last for. My twins were prem and teeny tiny, but by 4 months they had had the mother of all growth spurts and we're on he he 50th centile, and at 7 months dt2 is pushing the 91st. I couldn't have carried him for any significant length of time beyond about 3 months weight wise, (and not before that much as I had a c section). At that point you may not want dc1 on a buggy board (wouldn't really trust dd even now) so would need a triple anyway. In which case you might as well have one from the beginning and get most use out of it!!
Sorry for the great long essay, good luck with twins. It's been a very hard year, but I'm now seeing the benefits of having 3 so close together. Dd is starting to take more of an interest, and there's nothing better in the world then watching my boys giggle and grin at each other, trying to shuffle towards each other for a closer cuddle etc.