I was surprised to find that a squad of 5 sternguard with 2 lascannons costs less than a squad of 5 devastators with 2 lascannons. 2 units of 5 sternguard with 4 lascannons would cost the same amount as a 10 man devastator squad with 4 lascannons (since the devs pay less for their 2nd 5 guys.
Devastators pay a lot less for other guns, so sternguard are only cheaper if it's lascannons that you want. I do want lascannons though.
2 units of stenrguard would use up 2 elites slots, though if you wanted you could make them into 10 man units and send the other guys off with combi weps or something.
It seems to me that the sternguard are far better. They are all veterans with an extra attack and ld9, and the guys who don't have lascannons get special bolters (which to be fair they won't fire all that often).
You can make the sternguard into scoring units if you take pedro as well, which is a major advantage over the devastators. They would be great for defending your objective in take and hold games.
5 guys with 2 lascannons is a fairly effective set up and sternguard do it better. Why take devastators?