Double buggy or buggy & sling?

Matt will only just be 2 when this bubba arrives. I was thinking about just using the buggy I've got now to push him around & using a sling for the newborn instead of forking out for a double buggy.
Will this be practical?? How easy is it to walk with a baby in a sling while pushing a pram? I've never used a sling for a newborn before.
How long did your toddler stay in the buggy for? Matt likes getting out & walking, but is happy to sit there & get pushed around when we go for walks outside. He hates sitting in the pushchair when we're in shops though. Would he get the use out of a new pram or is he likely to be walking everywhere soon???

I had Hannah and Briona in a stroller and Kryssi in a sling until recently. Briona does like to walk a lot but when we go for a good walk around the block it's a bit far for her. A sling is great for the early days, when the baby is 6-8mos they get a bit heavy! and it's nice to have a double stroller. Recently as Kryssi approached the 9kg mark I went and bought a cheap double stroller from The Warehouse. It doesn't recline, so it's no good for a baby under 4mos, but it does the trick now that Kryssi is heavier and it was just $90.

We got the phil and teds double pram and its fantastic! Baileys loves it cause hes 'up high' :D