Right;
Uppababy - rumble seat is small and very upright, it's also very slippy, only the straps kept dd in place when we tried it. She thought it was hilarious though. There is no way to fit it with the bassinet nor with the seat fully reclined, not that the seat is suitable for a newborn either.
You can use it with the car seat for the baby, but depends on how long you're happy to have your newborn in the car seat. I do long dog walks as well as use the car so wasn't an option I was happy with.
Navigator - has been brilliant, really comfortable ride for dd, she refused to get into the vista after being in the phil&teds. She is fine in the double seat still up front, she is now 28 mths and on 50th centile for height/weight (about 12kgs at the moment).
DD2 is now almost 5 mths, 75th centile and is not happy in the lie flat position anymore, so am about to swap the chairs round. However, she's not yet sitting, and often sleeps on our walks so I'm a bit stuck at not being able to recline the main seat with the double seat behind.
DD1 is also now getting fed up of having to be in the seat and wants to dwardle walk everywhere, so...I have now just ordered the piggyback board for the vista.
She can fit on that for walking along roads and then can be free elsewhere and DD2 can be reclined in the seat. Plus I get my massive basket back!
They have both been brilliant but useless at certain points. Agree if you can get a side by side it will be much more versatile. And should do better recline/lie flat options.
So, vista wins on sunshades, basket, size of bassinet, better brake pedal, non puncturing tyres, easy up and down big steps, better suitability for the ages I now have.
Navigator wins on newborn/toddler stage (admittedly a tiny part of their lives, but I couldn't have done without it, mine don't mind a sling but for 3 hrs a day...no) easy to fit raincovers, comfortable seats, the newborn bit is cosy, dark and flat - she has slept brilliantly in it, except for when she has just got too big.
Navigator is lower/slightly smaller, more adjustable handle but you will get punctures, but is very light to handle, though getting it up big steps and balancing it on one wheel up and down kerbs as been a bit trickier than doing the same with the vista.
Both have been fine over fields and through floods. Actually the vista's front wheels really gathered up the mud and leaves on the towpath and needed clearing which the navigators don't.
On balance (and all being well with the buggy board, it's the vista I'll keep long term). I think 
Sorry for the essay! My advice is to go and try them both in varying position, kiddicare and John Lewis have both and the rumble seat available to try.