I am a stroller addict. I have both of these strollers.
I had the BJCM dbl first. I LOVE it.
Pros:
- Pushes so easily over all terrain
- Folds/unfolds VERY easily
- LARGE canopies
- Still fits my DS6 well
- Very study/well built
- Lightweight for its size
- Love the orange color (wish that wasn't the rental color now, though!)
Cons:
- Why oh why can't it just have a hook to KEEP it folded--a PAIN on the Dis busses, but DH rigged up a strap to keep it closed
- Basket underneath is big, but a pain to use
- No car seat adapter (there wasn't when I needed it anyway)
- Big when folded
- No cupholder or parent tray. They make one, but you have to buy it separately and it is NOT cheap, we bought one and liked it so much that we bought another one for the other side. $70.00 just for parent trays is ridiculous, but they really fit well (I bought a cheap one, but it didn't work nearly as good as the one BJ makes for it)
- My kids hate the way that they are always at a slight recline in the seats. When they were babies, it was great, but now they do not sit as well in this stroller
Peg Perego Aria Twin. I LOVE it.

Pros:
- Very, very lightweight
- Folds/Unfolds easily
- Folds nearly flat
- Carry handle
- Locks, to keep it folded
- Pushes very easily on pavement/ or at the mall
- Decent canopies
- Large accessible basket
- My twins LOVE having the front trays. There are very few side by side strollers that have these. This is what made me get the Peg over a Combi or Mac. They aren't real big and don't really hold a cup or bottle, but for cheerios and snacks, these are GREAT.
- It has a near flat recline, BUT also the seats can be full upright, unlike the BJCM, this way the kids can really sit up and look around.
- It comes with a cupholder, even if it is wimpy

My cheap parent console I bought works great on this stroller though
Cons:
- the handbrake is weird (I think the new ones might be different, mine is a yr old)
- the freedom straps are a little tricky, I like that they can move around but it seems like a lot of straps when I'm trying to get two almost 2yr olds to sit still

- does not push easily on grass or on gravel
- the basket could be a little stronger, they do have a piece of plastic in the bottom now but it should be a little stronger--I still pack mine full though!
So, overall I really love them both, I use the BJCM almost everyday walking around my neighborhood, but I don't keep it in my car. I keep the Peg in my car and always use it at the mall/outlets, etc. The Peg is now my Disney stroller though, DH said it was so much easier on the busses/transportation.
Hope that helps...but, I really do LOVE them both!
BTW, have also had the Graco Duoglider, that thing pushed well, but it was a monster of a stroller--liked it because I could put the infant seats in it though and had cupholders and a HUGE HUGE basket. Also have a Jeep Umbrella stroller, good in a pinch, but if you are not short you will be very uncomfortable pushing it.