Having driven all the cars listed I can say that they all work well and suit some people better than others (I for one never got on with the BJ4WE but find the B44 acceptable). My choice is the FS just because the balance, as Neil said, is superb. The FS doesn't punish like some of the others if you over cook it or get a dose of the 'red mist', just an easy car to push.
All those cars that you mention are going to mean you have to change front wheels (unless you already have a ZX5-SP ... I'm assuming not as you just said ZX5) but only the FS allows you to use your existing rear wheels, springs, (some) spares etc.
When you say the FS is £100 more than the other 2 I have to disagree, not seen many full spec CAT SX much under £300 and the lower spec cars are not comparable to the FS. The B44 is cheaper to buy but if you look at most of them there has been a fair chunk of money spent on "hop-ups" to deal with the lighter LiPo cells.
Nick runs his FS with next to no extra weight in and certainly no expensive 'Lipo tray' style weights, Ian Purnell runs his with just a little lead in the centre spine and now the season is over and it's experimentation time I will be trying to run the car without extra weight again too. Last year I was nearly there but not quite.
One of the advantages of the FS is the amazingly well sealed drive train, I have only rebuilt my diffs once this season and to be honest that was only because I cocked up and broke something so had to replace it!!! No RC car is zero maitanence but the FS is pretty damn close, try that with a CAT SX
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I think the summary is try an FS, I think you will like it ... if you do buy one, build it as per the instructions or Nick, myself or several others will be only too happy to share set-up info if you prefer. Nothing else needs to be bought that doesn't come in the box, even teh kit springs are a good starting point!