In my very limited mod experience (Tikka XP
http://www.candlepowerforums.com/vb/showthread.php?t=154135 & LongBow Micra
http://www.candlepowerforums.com/vb/showthread.php?t=168096&page=2) the SSC P4 is an easier fit because it is nearly identical in size, shape, etc. to the Lux emitters currently sitting in most of your existing older lights. The whole head of your light is designed to feed power to and position the LED correctly within the reflector or in relation to the optic being used. Replacing it with a brighter LED that has the same form factor is, I think, the quickest and easiest bang for the buck, especially when you're like me, and uncertain of all the details, equations, and specifications that actually went into the making of the original light.
Go to the LED manufacturer's websites and find the PDF spec sheets to see the dimensions firsthand (I think I got the right ones):
CREE
http://www.cree.com, or here:
http://www.cree.com/products/pdf/XLamp7090XR-E.pdf
LUMILEDS
http://www.lumileds.com, or here:
http://www.lumileds.com/pdfs/DS25.pdf
SEOUL
http://seoulsemicon.co.kr, or here:
http://www.seoulsemicon.co.kr/_homepage/home_kor/product/spec/X42180.pdf
To help toward answering two previous questions:
Thermal Pastes: as CaveDave said, you can reuse the ample paste from off the backside of the original star, or do like I did and use a bit of Silver 5 I had left over from a PC repair. I think the paste is only being used to transmit heat to the sensor (that little resistor looking thing behind the star attached to the two long silver wires) that Petzel uses on their lights to shut them off and protect them from overheating when used in boost mode. The three Arctic thermal pastes each have different properties for specific applications, but for the purposes of this modification I don't think it really matters … just don't slip up and get the epoxy unless you want your mod to be enshrined for all eternity!
SSC & Reflectors: until fairly recently most of our high powered lights have come with some sort of Lumileds (Lux) LED. The pattern of light distribution from the phosphor source, as well as its position within the well of the reflector or in relation to the optic, determines the characteristics of the beam shape, angle, hotspot, flood, transition, etc., that you see out the front end of your flashlight. I think that Cree LEDs work different in these respects and perhaps need the supporting hardware to be laid out different to match, while Lux and SSC are close enough to be quite compatible. There is enough difference between even the Lux and SSC emitters for manufacturers such as McGizmo to offer slightly different reflectors (ex. McR18 for Lux vs. McR18S for SSC), but if you're less than a perfectionist you can probably make do with either.
In the specific case of the Tikka XP (don't have the MYO) the original LED came on a star and the Seoul (SSC) comes on a star. The solder points, heat disbursement, front and rear holders that position the star/LED assembly, etc. were all designed around this specific size and shape device. Perhaps someone more innovative might take the time to fiddle a CREE into place and make it work, but I suspect the difference in effort is something akin to removing a splinter vs. performing surgery, and all for a negligible difference in output (see chart
http://img529.imageshack.us/my.php?image=fluxvfig5.jpg for U bin SSC vs Q5 bin Cree)
There may be smaller, brighter LEDs out there, but for my time, effort, and knowledge level, going with the SSC just works out better. Take the easy road and feel good about your first mod … and feel free to correct any mistakes in my understanding of these subjects.