I have 16 months between mine, DD1 was not walking yet when baby came along and the double was really useful, I wouldn't be without it to be honest.
If we were just going to the local park I put baby in a stretchy wrap sling (not a BabyBjorn!) and put the older one in the single maclaren and it was great. I'd have my hands free to get DD1 into the swings and things. I did love my sling, now use a Mei Tei instead of a stretchy wrap.
That being said, if we were going on a family day out or doing things where we would be in and out a lot it was so much easier to use the double (a phil&teds). I found that if we were going into town say to meet up with some other mums for library and then coffee it was a faff to get baby in and out of the sling for the library and then again in the cafe or just so I could take my coat off , even worse in the bad weather.
We did quite a few days out to the zoo and that sort of thing too, where I think we would have struggled without the double. Plus, in full torrential rain I think its nicer to be able to get them both warm and snug in the buggy under the oxygen tent, knowing they'll stay warm and dry. I don't have a babywearing coat so baby always got a bit rained on in the sling.
They are now 2.5yrs and 15mo and I still use the double sometimes, the single maclaren and a buggy board sometimes and a Mei Tai sling at other times.
If you can afford one I would get a double, maybe get a secondhand one that you can sell on to recoup your cost if you end up not really using it?