I had a tadem pusher with my older two children and found it heavy and it was hard to turn around corners. I had a side by side for my younger two and couldnt get into some shops that i found annoying. If you shop around there are alot of pushers that have toodler seats that can be clipped on the front. Ask mums at the shops with double pushers if you can look at their pusher to see what they are like

I've had two side-by-side prams.  The first one was really sturdy, and very easy to push (single wheel at the front that pivots).  It fitted through the wheelchair access aisle in the supermarket, no problem.  I used to put two kids in the front, and one on top.  It was stolen from the front of my house, because (by a whisker) we couldn't get it through the front door without folding it up, and I was too lazy to fold it up every time I used it.

The second one is a little narrower, and not very well balanced.  It's still easy to manoeuvre (single wheel at the front that pivots), but it overbalances more easily, and the wheel gets a good vibration up if I try to go faster than a medium walking pace.

If you do buy a side by side pram, test it with kids in it first, if you can, just to make sure you get a sturdy one.  I'm sure we just managed to get a lemon.  The pram is quite heavy if you need to pick it up to load it into a car, but to push, they're fine.

I have seen the side by side prams, and yes the fit on the escalators, but not many fit thru the checkouts. If I were you I would try and get a Tandem pram you can get ones that both seats lie back. Im not sure about the ease of pushing tho.