Yes, the issue with the arguement is that it boils down to whether you place more emphasis on the "attempt to cast was successful" or the "the spell was successfully cast" bit. Unfortunately they both mean different things, and I believe both phrases are contained in 2 separate sentances within the same paragraph (further muddying the issue).
* For a spell attempt
to be successful, the casting value of the spell needs to be met, otherwise the attempt is unsuccessful.
* For a spell to be successfully cast
, it needs to be both attempted successfully (ie: met the casting roll) and not be dispelled.
It seems we are not alone in this conundrum, as this is causing quite a stir on a few other forums as well. Warhammer.org for instance seemed to agree on one interpretation and then changed their minds a few days later, so the rule is far from clear.
I think this is one of those rare cases where we will just have to 4+ it during friendly play, or let the TO decide for themselves how this is going to be played out at events. Personally I have my own opinions on End Times rules and competitive events, but I think thats already been covered elsewhere.