Okay this is not a whinefest post. It's my breakdown of the two troop choices.
SM tactical squad:
You have to take a full 10 man squad for this unit to work. You get a free missile launcher and flamer plus you can throw in a rhino for 35 points. The flamer is nice but I am not a fan of missile launcher, lascannons are dirt cheap for tactical squads and this is the best place to take them. A lot of players feel that the power fist is not worth the points but I disagree, 10 Marines and a power fist can take down just about anything. Space Marines can split into combat squads and have combat tactics. I am not a fan of splitting into combat squads. Ten Marines are good at holding an objective but five will get swept aside; it's just that simple to me. Combat tactics are situational at best. There is always the chance the squad will not fallback far enough and remain broken because of a nearby enemy unit within 6". Sure voluntarily breaking is better than nothing but I don't see it winning games. Maybe you'll get lucky, fallback from a dreadnought then destroy it with your meltagun. That brings me back to the flamer, it's definitely a good special weapon but with all the mech running around now I'd much rather have the tank busting gun.
So here is my optimal tactical squad:
lascannon, meltagun, power fist - rhino
I'm not a fan of razorbacks. You get a great bargain for a 35 point rhino. It's got firepoints built in and can repair itself. The razorback you are paying more points for a tank that can easily be destroyed or shaken losing the advantage of the gun. Razorbacks lend themselves to splitting up into combat squads which is bad. The build I listed above is the best combination for the points and brings out the versatility of a tactical squad. Remember that the lascannon has twice the range of a multi-melta and can penetrate AV14. The advantage of placing lascannons in your tactical squads is each unit can fire at a separate target so there are no wasted shots.
IG veteran squad:
You get ten veterans that are BS4 plus can take 3 special weapons plus one heavy weapon. I'll start out immediately with my optimal build:
3x meltagun, lascannon, power fist - chimera/multi-laser & hull mounted heavy flamer
Another great build is as follows:
3x flamer, heavy flamer, power fist & demo charge - chimera/as above
If I were to run IG I would run four to five veteran squads. Three to four would be armed as per the first build and the last squad as per the last squad. I would also field Straken to get the most out of the power fists. Two squads of charging veterans with furious charge can generate a lot of attacks. Sure you don't want to play these units as assault squads but it's there for the contigency.
So what is better, the Marines or the guardsmen? In my mind the vets are much better hands down. They are cheaper so you can field more and that helps to make them better at holding objectives. I think vets are much better at taking objectives. They can roll up with the chimeras, hit you with the heavy flamers then blast you with the meltas or more flamers.
I have added the lascannon so you can pillbox. You have decided to castle up and trade shots with the opponents. A BS4 lascannon is great thing and beautiful. If you have four they are going to do some damage. I have added the power fist so the vets have a bit of punch in close combat. If you take Straken as I advice these squads suddenly give you a cheap counter assault element.
Finally let's close with shotguns versus lasguns... I am all for the shotguns. You don't want to disembark your veterans unless you have to. The shotgun allows you to fire twice and then charge. The shotgun is the synergistic choice plus it looks a whole heck of a lot cooler.
So in conclusion I think vets are a much better troop. They've got more better guns plus their transport is a lot better. In many ways vets are the IG equivalent of a tactical squad and this unit is the biggest change to the army. Platoons have always been there as a troop choice. It's obvious that the switch in slots between vets and stormtroopers was a big boon to IG in general. In fact you never see anyone take stormtroopers anymore. Last but not least vets also have a lot of nice options to add more punch and flavor.