Hi Kaleem,
This is very generic questions for which answers will differ from person to person.
Coming to say which one is easy, I would say all are at same level of competency. Only difference remain in add-on functionalities specific to EHP version.
All certification codes you mentioned in your query has only difference of EHP versions. As a ground reality, its always good to get certified in latest version. So its better to go for certification in C_TPLM30_67. ( SAP ERP 6.0 EHP7)
I believe, you can not purchase OR free download SAP official material unless you join official course with SAP or their training channel partners. So Just google for PLM300, PLM305, PLM310, PL315 PLM316 & PLM318. & refer release notes from help.sap.com for understanding add-on features/components for EHP 7.
Hope this helps.
Regards,
AMol