The In Car Safety Centre people (particarly Louise and her mum, Linda) are so so knowledgeable and will be A&E to talk you through all the options. You can order from them to send the seat by post if you decide that's the right option
I have bought/chosen many ERF seats from them, for our two cars, my DMs car and my DSis car. From my extensive (though amateur) research, and many visits to the ICSC, I can add a bitto your thinking:
BeSafe have several seats that are virtually the same. We have two BeSafe IZI Combi ISOFIX X3, which are great seats. The IZI Combi Seatbelt X3 is the same, but non-ISOFIX, obviously. The seat has lots of side protection, lots of leg room (because it has a brace that pushes into the back of the rear seat, holding the child seat away from the backrest of the main car seat), and it is high up so the child can see out of the windows :) On the other hand, the leg is big and chunky, so it's hard to put much luggage in the rear footwell (May not be such an issue if you just have one as you have the other side?), and the polystyrene of the seat comes well forward of the hard plastic casing, so you have to be careful not to lean on the side of the seat so you don't snap it. The seat is built such that the driver/pans sender front seat has to be touching the child seat when it is rear facing, so it is a faff to change the driving position (DH and I have to both compromise so that seat can stay put for either driver!). I use the BeSafe rear facing in our main car, and occasionally forward facing in out second car. As far as I know, the IZI Plus is the same as the IZI Combi (slightly different brace, and may be internal differences), but is seatbelt fitted and so can be used rear facing until 25kg, though I see you are concerned about having a forward facing option too.
The Britax Two Way Elite is a really versatile seat. It can be used rear facing or forward facing to 25kg with the 5 point harness, and fits in most cars. It is the only seat (I think) that you can use a 5 point harnes forward facing to 25kg. The disadvantages are that the front seat must touch the child seat (so hard to move driving position), and there isn't much side protection/head support. It was the lack of side protection that made me chose the IZI Combi over the TWE when we bought seats for my 1 year old twins. Some people say that the tethers make it a pain to move the seats between cars, but to be honest, I find the ISOFIX IZI Combis harder as they are bulkier seats and there is a foot prop etc. We use the TWE seats forward facing in my mum's TT (only Group 1/2 seat we could get in the back of a TT, in case anyone knows of someone struggling to get kids in a TT!), forward facing in our second car, and rear facing in our main car.
The Klippan KISS 2 is a really versatile seat, though it is bulky and wouldn't fit in our cars (sister had it). It can cover a wide age range, but I think it might be ISOFIX only.
The Klippan Trofix was also a contender for our seats. It was a nice seat, though I think ISOFIX only until 18kg.
I have tried to give some info on how we find the seats to use, but the ICSC people have so much more knowledge. They're really friendly, so give them a ring!
From what your said, I think the Two Way Elite will be your best option, as it can be use rear or forward facing, has a 5 point harness to 25kg, fits most cars, is light and easy to move .... a great all rounder really! My only reservation would be the side impact protection for your younger child, but it has passed stringent tests and been approved for 9kg plus.
Good luck in your search (and I hope you escape becoming quite as obsessed as me!).