It is better to keep the front 3 speakers at the samelevel if possible, but if that's not possible, it's not the end of the world. In some situations it won't affect things too much (as in Big Boss' system). But if it does, you could tilt the centre up by using some rubber feet underneath the front of the centre speaker. There are some centre speaker 'cradles' on the market, a sort of tabletop type stand allowing you to tilt the centre. There's always a way around everything......
As far as Radius vs Apex goes, Apex is designed as a higher end product than the Radius. I think you'll be happy with either, but the Apex range will have several advantages in terms of it's cabinet material and higher quality drivers, which are derived from the RX series. I've heard them, but only at the Bristol Show, so there's no way I'm passing judgement until I hear them properly in our own demo room.