I'm about 5'10", due to upgrade to a SC superlight, according to the sizing charts I'm inbetween medium and large sizes for best fit.
Currently riding a 17,5" Fisher hardtail with Genesis geometry.
have tried a medium for size and it felt fine, might be better to get a large with a shorter stem though, especially for longer rides.
Any ideas?

I'd say med - I'm 6'2" and happy with large,

I'm a smidge over 5'10" and bought a medium. I'd tried a friends large Superlight and could ride it well enough, it just didn't give me much room to move about and I felt a bit perched and streched on it. Within a few minutes of getting on a medium I knew I had the right size.
I should mention that experience has taught me that I (i.e. my back and shoulders) am more comfortable in a fairly upright position on a bike, so I tend to go for frames that have a fairly specific top tube length of around 22.5".
BTW, I can recommend the 5th Element Air shock - a noticable improvement over the regular Fox Float R.

Im also 5-10 and I have a medium Blur, I think the sizes are similar. Go for the Med.

I would have a look at the sizing chart - if you like stretched out the large is fine - I have a large (I'm 6-1 with long body, shorter arms and legs) and my smaller frieds who ride a 17.5 Marin FS thinks its fine. I need to ride the large with a 130 stem - I would consider the XL if I bought again. If you can get a large for a longer ride - try it out. That said - I demoed a medium - which was still a fine ride

Re the 5th - I have got one for my wife's Marin - bit improvement in handling etc

Go for the medium,i`m 6`2" and i was in the same dilema as yourself.I was advised to go for the XL, but when i did try one it did`nt feel right,so went for the large.Well pleased.