I have all three of them.
pure sound quality [regardless price and comfortable issures/cord.]
YUIN is best, CX300 is slightly better than EP 630. EP 630 has more bass overall but not so punchy as CX300. both EP 630 CX300 give you feeling you are in the narrow room with the music. YUIN PK2 makes music free and open. YUIN PK2 has way better sound stage[not that big as you might image from all the ravings], you could breath with the music, but weak in bass, too much in mid, great accurate in treble(no sibiliance as V6) and very warm forwarding timber. Although PK2 performance is still weak for classical music, it's great in regular soft/hard rock/trance/rap/jazz, which need more mid range emphasis. it makes you feel you are RIGHT IN the music
comfy:
EP 630 is winner, Y cord, CX300 is similar to EP 630, but annoying J cord. PK2 is less comfortable to ware, imagine solid earbuds stick in your ears, plus J cord, staight plug as EP 630.
Looking:
PK2 is ugliest. no argument in this section.
Count in the price tag.
If EP630 is for 20CAD, the CX300 for 50CAD and the Yuin PK2 for $80-85CAD.
I will buy EP 630 again and again without any hesitation.[make sure you get real EP 630 with retail version package, not those knockoff on ebay.] since it is good as CX300 but less than half price, it's stealing. it's so comfy that I sleep with it on all night.
After you have the EP 630, and still have extra money and want to bear the uncomfortable issure for good sound quality, buy PK2. but I bet you will switch to EP 630 for regular everyday listening.
"These will mainly be used on a bus and in an outside environment with minimal walking but it would be good if they could be used while jogging."
EP 630/CX 300 has much more isolation than PK2, if you will use it on bus, EP 630 is the winner. for jogging, EP 630/CX 300 is obiviously winner again. PK2 pops out while I am jogging, it's only good for walking.
"Are they worth the extra price over the other two?" for pure sound quality, the answer is YES, if you count in comfy/looking/convinence, NO.
Overall, EP 630 fits you best. Hope that helps.
PS: let us know where to get EP 630 for $20. I need some backups.