I have used both, but rather than proffer an opinion at first, I'll suggest there are some significant differences between these saddles- most notably, in length. The ISM's are generally truncated saddles where the nose has been amputated and they are configured to "sit off the nose". The logic may be, if the nose is bothering you, remove it.
More recent ISM saddles trend toward a more full-length, comventional orientation- not to replace previous designs, but as an addition.
Cobb cycling saddles generally trend toward a more conventional 27cm length.
A longer saddle may provide the rider with the benefit of greater surface area and more locations to sit on- you have more room to scootch forward and back. The shorter saddles may not offer the same degree of positional latitude. You basically have one location to sit, or a reduced surface area to sit on.
And, as an addendum, whenever I see people asking about saddles with an "unconventional" or otehr than traditional design, I always list the saddle comfort influencing factors, in what I'll suggest is order of importance:
1. Good bike fit and position: Is your saddle height, angle and fore/aft optimal?
2. Good saddle acclimation: Have you "got your seat" as the Euros like to say. Have you been on the saddle at short intervals regularly for at least 6 months?
3. Are you over weight? Being over weight increases saddle pressure and creates a catch-22 of needing to ride to lose weight, but being uncomfortable on the saddle because of being over weight. Losing weight will help enormously. In the mean time, a saddle you can tolerate in the mean time helps too.
4. Good bike shorts (not tri shorts for training) worn tight enough and without underwear. People who spend a lot of time in the saddle like bibs. They hold the pad snuggly against your body and prevent shifting of the anatomy.
5. Use chamois cream: Assos, Chamois Butt'r, Dee's Nutz, even Nivea (as its used in Europe a bit). Some type of chamois lubricant helps enormously.
And finally, since you asked, having ridden both I am a fan of the Cobb Cycling saddles and the Fizik Arione Tri and Road saddles.
Tom Demerly
The Tri Shop.com