I had one with 2.5 yr age gap and got rid asap (it was too wide for most places including own front door) Then had a good pram with buggyboard for ages, about a year. Eldest is fairly generously described as non compliant and he was trustworthy enough with a board at 2.10ish. With 2y1week age gap I have done it both ways, right now for our lifestyle a double is the way to go, the 2yo couldn't for practical (stopping for 5 mins to inspect contents of drain, walking in the wrong direction, any number of other toddler distractions etc.) and tiredness reasons walk everywhere and is not yet trustworthy enough for a board.
If you plan on keeping the same pram you currently have then a double is potentially a waste of money purely because you'd need to buy something else. If you're buying something else anyway then that argument carries less weight. I didn't have a pram that could carry either child at any given time (not very easy to find if you want them to face you, easy if you don't) so whilst I like the carrying option I would still have had to get something different. Also not a fan of carrying full time really, dunno why, just personal preference.
Suppose personally I feel like 2.5yrs is a really inbetween kind of gap, it really could go either way you could find it totally manageable or totally unmanageable. I have def found one more vital now we have a schedule that exists separately from the younger two, school etc. etc. With the first two we had more time to look and the sky, smell the flowers and look in all the drains we felt like!
It's not an all or nothing decision anyway, you can always sell on (ebay, preloved, gumtree etc) if you don't like/use it without necessarily losing a lot of money. It just depends what you buy and how well it holds its value.