I have some Mavic XM 317 wheels on my five but think i could do with something burlier for my lack of skills and weight 15st. Most of my riding is fairly aggressive XC trails off drops and jumps and mild DH. Want something wider too.

I have a chance of some Stans ztr Crest on red hope 2's, would I be better off with Flows than crests?

I ride 355's basically the Crest before the name change. They are on my 5 with Pro 3 hubs 24 spoke SP jobbies, they're fine for all the Enduro xc stuff I do but wouldn't personally want to be doing DH or big drop offs on them, that said the Pro2 version with more spokes may be up to it. I was 15st ish as well dropped a bit now though.
If your not worried about the weight to much the Flows will take pretty much all you can throw at them and potentially you can run wider tyres on Flows.

Crests are actually wider than 355s, and indeed Archs, they're very close to, if not as wide as, Flows. If you don't care about the weight then the Flows are a more sensible option!

njee oh tome of all tyre/wheel knowledge – about to get a rear Hoop for wife's Rockhopper. We both run narrow (1.8") tyres – Is a Crest/355 going to be too wide? One thing that attracted me to the xc717 was it's narrowness.

She's lightish (9ish stone), pretty fit so hardish on gear and a bit mad on DH stuff.

A Hoop seems a bit OTT on a Rockhopper but it's just such good value it seems daft to rebuilt the old rim onto a new hub. She also loves the Hope clickness of mine.