Definitely go the MB. I have a single and it is an awesome walking pram. When I got a double when DD2 was born I didn't go the double because I wanted two - one for the car and one for walking and I just couldn't justify the then enormous purchase price of the double MB. I ended up getting a double valco, which is a fantastic walking pram too - I still use it occasionally for my 4 and 5 year olds.

If you have the donkey so aren't thinking of having to get a big heavy double in and out of the car then go for it. Nothing beats them for ease for you and comfort for the kidlets - decent sized seats so your 3.5 yr old will be comfy, individually adjustable etc and the ride is lovely and smooth. I also find the weight of the double is nowhere near as much of an issue pushing as I thought it might be.