We had the powertwin with oldest 2 (dcs were newborn and 16months and used until eldest started walking everywhere at 3 1/2 but can still fit at 4) which I found was fantastic. There isn't a perfect thing out there but I think the powertwin comes really close. It is heavy to lift but perfectly possible - I lifted ds up and down the stairs (2 flights) to my flat every day in it until he was over 2.
Pros - It has tyres rather than foam wheels so gives a smooth ride and comfortable push when weighted down heavily or with different sized children. The newer (profix) model means a car seat can be attached and removed with one hand which is fab. There are several car seat options which can be attached for the baby (as well as the possibility of the rear seat from birth) - matrix, rebel, strata, carrycot. The wheels release easily (once you work it out) to give it an extra helping hand into the boot (I once got a lift from my friend in her micra - the micra ended up being successfully crammed with 2 adults, 3 children, the powertwin and two single maclaren type buggies). You can walk for miles and miles with ease over different types of terrain. It is the width of an ordinary single pram and not very long - me, my sister, the powertwin and a massive suitcase fit easily into what I am sure is the tiniest lift in the world at liverpool central station. The disk brake is really handy if you march everywhere at speed like I do and need to stop quickly or need to keep the pram still while waiting to get off a train or bus. Turns on a pin due to the swivel front wheel. You can fix the baby in the car seat above the front (not from birth seat) if you want the toddler to be able to see the baby or find it more comfortable to push that way. The shopping basket, whilst not massive, is useful and coped well with the huge amounts I used to stuff into it. If you use a car seat attached for the baby you can use the gap between the car seat and the pushchair seat as extra storage space.
Cons - Punctures will cause extreme distress and are a pain to fix so put some puncture protection stuff between the tyre and the inner tube as soon as you get it (ordinary stuff for bikes will do). It is not hugely stable when getting onto and off buses and trains as it is a 3-wheeler but is fine with kurbs (actually this is just nit-picking, once you get the knack of getting it on and off it's fine). If you have the baby in the seat (without the car seat) at the back the difference in weight can make the pushchair pull to one side because of the swivel wheel. The pushchair seats are not massively roomy but are adequate even for large children that said, I'm yet to come across a double buggy which is roomy. The child who is in the front tends to put their feet onto the front wheel as the foot rest is quite small and high but I got around this by using a large footmuff and keeping it zipped up a bit at the bottom during the summer so the feet are not tempted to wander. The disk brake needs tightening every once in a while. The profix car seats are not compatible with the older powertwins ( and vice-versa - new profix powertwin will not take old car seats) that need attachments to add the car seat so watch out for this if you buy secondhand. The plastic that holds the rear brake together can get stretched or ripped (renders the brake useless) if you push the pram with the brake on so be careful to check the brake is off, the little mechanism at the front which locks and unlocks the front-swivel function often loses the little plastic switch so can be a pain if it gets knocked and locks as you have to attack it with pliers and can also sometimes be a pain to turn (it sticks and you sometimes just end up pulling the plastic bit out) but the swivel doesn't lock unless the front mechanism gets a knock (usually a kick from dangling feet in the front seat). The brake sometimes drops slightly during use and makes an annoying but harmless clicky noise. My powertwin is the older model however and they may have improved some of these things with the new one but none of the things are major problems to me anyway.
I walk everwhere (my husband drives) so it has proven to be a very good all-rounder. It is easily adaptable to walking, public transport, the countryside, towns and cities and car travel but I think if I mainly went places in the car and didn't need to be able to carry anything in the shopping basket I'd have a look at the Phil and Teds mainly cos it looks smaller and might be easier to get in and out of the car (don't know if this is true). I never have because I wouldn't be able to do things like my weekly shop if I didn't have a shopping basket at all. It looks and sounds good although I'm unsure about the dangling 'cocoon' for the newborn - I liked being able to look at and talk to my baby while we walked.
I'm sure you could pick either up on ebay after test driving at mothercare or somewhere else. Maybe you could ask mothercare to send one for you to test (without obligation to buy) as you are interested in trying them out. Lots of shops will do this but I've never tried with mothercare. Failing that, stop people in the street and ask them about their pram - lots of ppl stopped me with the powertwin when I first got it and I never minded letting them have a push. Hope this is helpful!