The Bolide strap really isn't cross body; not long enough for me at any rate. I have the 35 and love it and yes, the zipper does come in handy in crowds, when out, etc. The strap at least gives you that shoulder carry option that the Birkin does not and there are many places I would not feel comfortable bringing my Birkins. Work, for one. So those are things to consider.
As for resale, I find Bolides priced much higher used than they used to be even a few years ago. But new they are pretty close to the cost of a Birkin 30 so there is also that to consider.
Thanks mistikat! Definitely good thoughts to ponder on.