*I think* the only difference between MKi and MKii in the CX--- range is that MKii has in-line volume control
If you like the CX300 then I would not advise you against buying them. They do the best job out of all the budget canals in that they manage the thumpy bass sound better than the rest. I would much prefer the MX series, fond memories of my old MX500 here, as unlike the CX series they do not have uncontrolled bass all over the place
Sennheiser are strange. PX100 is great. CX uncontrolled. Sport series has no bass. Their IE offerings manage to compound the expansive sparkly sound with excessive bass
I used to own loads of cheap canals across several brands before upgrading to IEMs. My real advice would be how much do you have to spend? You have discovered the UE's, although you have formed your own opinion on the sound signature of that one model
The CX400 and CX500 get decidedly better ratings than the CX300. I have only heard the CX300 so could not say either way. JBL 220 and Denon C551 seem to get favourable reports in preference over the CX range