I have played the 740 and 550 and IMHO the 550 is better in the sound department, but the 550 does not have organ flutes. The 740 is older so I think the sounds are improved on the newer 550. The 550 has a sweet flute that the 740 does not, but the 740 has a classic flute that is nice. As mentioned before, the 550 replaces the 540. The 740 was a more expensive unit, so it's in a different category.
Not sure about the styles.
Kevin