Consider 242 and 252 to be the same machine; real difference between 242 and 550 is that 550 will take sra3 from the trays, although not extra heavyweight which will still need to be run from tray 5 (unless you have a high capacity feeder). 700 offers better decurling and registration and again will take SRA3 from the trays.
The price of consumables is another consideration; Drums for the 242 are expensive (up to £200 at the moment in the UK), but toner is cheaper (£130 for a twin pack). On the 700 you will be paying up to £80 for a single toner; a little less on the 550. On the 550 and 700 drums last a bit longer because there is a roller to clean the charge roller rather than a sponge pad. Fusers are much more expensive for the 700 and 550 than the 250.
So the answer is decide what you want to do with it and what is important to you (price, registration, paper handling etc..) and pick the machine that is appropriate to your business model. I would say that the following is a good guide based on what is most important
Running costs 242 (Drums expensive, but service items and toner are cheaper)
Compromise 550 (Slightly better registration and SRA3 from trays up to HW1 stock, Slightly more expensive service items and toner)
Registration 700 if installed and set up by an engineer (SRA3 from trays up to HW1 stock, more expensive service items and toner)
Image quality is almost identical although some people say that the 700 is a little flat compared to the others, I think that with the right stock it is negligible.