I could talk for days about the differences between these products.... But I won't!
First, I sell all of the three products you are asking about. However, the is40 has been replaced with the is35 a few months ago. The difference is it is now black instead of grey and it has the vocal harmonizer and guitar effects like the is40m module. If you need a good sequencer in the keyboard or module, your choice would have to be the korgs over the yamaha's because the korgs have microscope editing and all functions are carried out in the keyboard. The negative of the Yamaha sequencer is it must record songs direct to disk and all you can edit is quantizing. But, the Yamaha lets you record your own user styles and the korg can only load new styles from disks that are either made in a korg i30 keyboard or perhaps a sequencer program. Sounds in the korg can be edited where on the Yamaha they can only be edited with a program called XG works in your computer. As far as the internal styles are concerned, I think the Korg is more contemporary, but not that different. However the intros, endings and fills are much more exciting if you want excitment. Some readers of this forum say they prefer a more simple intro and ending. On the Korg these are pretty exciting. It's a hard decision. The US price of the PSR 740 is $1395.00 (list price), the Korg is40M is also $1395 (retail) and the is35 keyboard is $1695.00 (retail). The Yamaha is the only one that will also work on batteries. All three have mic inputs and display lyrics.
Kaye's Music Scene
Kaye's Music Scene (Closed after 51 years)
West Hills, California