Ten Storm Troopers w/ two plasma guns - Would be more. About 5.8. Easily the most powerful option, but also costs 195 points.
Ten Veterans w/ three plasma guns - Would also be more. About 4.3.
The plasma Veterans are almost as good as the basic Storm Troopers even without enemy cover. If the target Marines do have cover, the Veterans would actually be better than the Storm Troopers. The Veterans are also significantly better against monstrous creatures, and enormously better against vehicles.
If the Storm Troopers buy plasma guns, then they easily edge the Veterans out (except against vehicles), however they also leap from costing 50 points more to 80 points more. While the Storm Troopers do have better armour and delivery options, they also do not score.
It's also worth noting that Veteran squads with demolitions are a powerful counter to high-toughness monstrous creatures because whiele they will lose a few models to the enemy attacks, the remainder will all hit back with a S8 Ap1 meltabomb that probably only require a 4+ to hit. A Veterans squad with three meltaguns and demolitions cost 130 points and make fearsome big-game hunters as well as being a strong anti-tank unit. Plus they have a demo charge.
Also, by memory I believe ten Storm Troopers are 165 points basic.
EDIT: I think the one potential Storm Troopers have is in attrition. Looking at this kind of comparison, I would make best use of range and attempt to keep up a steady fire from the hot-shot lasguns over the course of several turns - a situation in which overheats would swiftly (and massively) degrade the Veteran's effectiveness. However, Storm Troopers struggle to operate from Chimeras because they can only use 50% of their firepower from the fire points, wheras the Veterans can use almost all of their firepower because it's focused on a few models. Normal casualties (e.g. from the schmucks you leave out in front) also degrade the Storm Troopers far more because each model is worth an equal amount.
Sadly, if you're going for a long-term attrition strategy to wear enemy units down, the answer is neither Veterans nor Storm Troopers. It's platoons with ranked fire and a ton of independently-firing Chimeras.
RETRO-EDIT: I guess to me the only realistic use of Storm Troopers in a non-suicide deep strike role is as a support unit for Al'Rahem's platoon. By definition Al'Rahem cannot bring along Veterans, and while his own platoon Command Section is a decent unit, it's also expensive and contains the only nearby officer (and one for whom we presumably pay quite a lot of points for all those better-than-usual orders). In this case a large squad of Storm Troopers outflanking at the same time, with an Astropath for good all-round reliability of reserves, might actually fill a worthwhile niche. If you then combined that Storm Trooper squad with 'Like the Wind' you could rapid fire enemies and then run back out of (probable) charge range. Would this all be good? No, but it's the only way I'm ever going to use a full 10-man squad and that's a sad thing for someone with more Storm Troopers models than the bloody Deathstar.