Seeing as Rachel mentioned the Klippan TrioFix, I thought I should chip in that they do another seat called the Kiss 2, which is also sold as a Graco DuoLogic. Rear-faces birth-18kg, fits either with a belt OR isofix, the infant seat can be used on the base or fitted with a belt without it, and has wheels

It has 3 recline positions. My eldest girl was nowhere near out-growing it age 3 (albeit she is a small child) and my littlest is very happy and comfy in it now. That might be an option, as it's also one of the most compact seats on the market. It's also super easy to move from car to car, as it doesn't need any tether straps or anything.
As for putting them in the front, this is only doable if you can turn off the airbags. We've done it a few times, most notably recently when we did a long journey down south. We put Midgelet in her DuoLogic in the front seat (airbags off) and Roo was in the back. It meant that whoever wasn't driving was available to pass things (snacks, toys, drinks etc) to Roo, and play with her/keep her amused on the long drive, without having to constatnly twist round from the passenger seat. And Midgelet pretty much slept the whole way, so she wasn't much of a distraction.
In practical terms, it didn't cause us any bother. I didn't find it any more distracting having her in the front seat than in the back. I have mirrors up when she is in the back which I check as frequently as I check my rear-view mirror anyway. All I did was sweep my eyes over her as I did a mirror check. I wasn't worried about her safety - if you have a read of some of the info on carseat.se they have a few great pages on why front-seat travel in a rear-facing seat is safe.
I agree with Rachel about the First Class Plus (or any combi seat at this point). You probably wouldn't get enough use out of it compared to how long an ERF seat would last to make it worth buying unless it was the only option.
Regarding the Mobi, I've never used one, but I think it's elli21 (?) on here who has one she got from the Glasgow Pram Centre, and as far as I know she is very happy with it.