@RiotScruffy: Need your opinion again for my pure-magic marksman

I've been getting some suggestions regarding her passive, and I'd like an expert opinion.

#Current passive

Mikoto’s basic attacks deal magic damage instead of physical damage, but cannot critically strike. Each 1% in critical chance instead gives Mikoto 1% penetration against bonus magic resistance (this includes MR from runes, masteries, items, and buffs), up to a cap of 30%.

Some people have said that this might make her damage too hard to counter, so this alternative was suggested.

Mikoto’s basic attacks deal magic damage instead of physical damage, but cannot critically strike. Each 1% in critical chance/critical damage instead adds 0.X% of Mikoto's AD to her basic attacks as bonus magic damage.

Which do you think would be more suitable for a marksman? Note that in either event, I'm having her ultimate provide her with a large amount of MPen against enemies that are in a zone she creates.