I have a Festool TS55 (the older model) and a number of colleagues use either the TS55 or the more modern TS55R, but I have used both the Makita SP6000K and the deWalt DWS520. I suppose it's down top personal choice. Personally, I didn't like the peculiar action of the DWS520 when I tried it, but the limited range of rails was also an issue (I need and use a 3000mm rail on floor work regularly). That left the choice between the TS55, the SP6000K and the Mafell MT55cc (you missed out that one). The MT55cc has certainly got a better rail system (which is shares with the Bosch GKT55CE - a sort of Chinese-made Bosch "clone" of the MT55cc, but missing a few features), but at the time I wasn't prepared to pay the £100 premium for the Mafell. Looking at the SP6000K and the TS55 at the time I had to conclude that there wasn't much in it (I felt that the Festool was slightly beter finished, that's all), however Festool offered (and still offer) a 3 year warranty with their tools which includes free on-site insurance - so if it gets nicked they'll replace it. That made it a no brainer IMHO.
The big difference now, however, is that there are several cordless options on the market including the Festool TSC55R and the deWalt DCS520T (54 volt Flex battery system) which may influence your decision making. There's also the Bosch corded tool (GKT55CE) but it is Chinese made, the same sort of price as the Festool and lacks the insurance.
In terms of these saws, none of them (with the sole exception of the TSC55R) work at all well without a vacuum cleaner attached - and that means a Class M unit at £450 or so for use on site (now mandatory) and not a manky old Henry - but as at least part of the reason for going for a plunge/rail saw is to have a clean working environment and reduce the clean-up at the end of a job I'd say that's worthwhile. If expensive.