I know you can take both. The question is, if you HAD to choose, which would it be?
At the same time I am selling off my Nids, I am building a new army consisting of Marines/Admech. I have decided to make a thematic/narrative force. I am calling it the Iron Revenants Strike Force (Iron Hands/Clan Raukaan). Based on the backstory, I have decided to go with only Scouts for troop choices. This is based on the disaster of the Gaudinian Heresy, in which Iron Father Kristos charged into a chaos infested world without the benefit of proper intelligence regarding the threat he was facing. Only the actions of then Epistolary Lydriik saved the Iron Hands from total corruption. With this in mind, on to the choice at hand...Scouts or Tac Marines?
Lets start with Tac Squads. In terms of cost, Tac Marines are a few points more out the gate. For those points, you get a 3+ armour save. You also get more choices in terms of dedicated transports, including Drop Pods. A ten-man squad also allows up to two Heavy Weps, including Grav-Cannons. Also included is the option to carry up to two Special Weps. This totals four Special/Heavy weps in a 10-man squad. That is potentially a lot of fire power for a 10 model unit. Finally, Tac Marines grant access to the Battle Demi-Company. Remember though, we are comparing units, not formations. Perhaps another time.
Scouts bring addition special rules; both Infiltrate and Scout. This gives Scouts a great deal of tactical flexibility, arguably more than a non-Drop Pod Tac Squad. This flexibility can be applied both during the deployment phase as well as during game play. In terms of dedicated transports, they are limited to Land Speed Storms. However, this option gives the Scouts even more ability to move quickly around the board and grab objectives; let's not forget they can Jink and mitigate that marginal armour 10. All Scouts can take Camo cloaks which improve cover saves. This, and appropriate use of terrain, can mitigate their 4+ armour. Which is better against AP3; a 3+ armour save or a 4+ cover save? Weapon upgrade options? For a 5-man squad, you can add Sniper Rifles, Missile Launchers or Heavy Bolters. 10-man squads add no more weapon options, so that is definitely one for the Tac Squads.
Vet Sarge upgrades are the same either way. Both Scout and Tac sergeants can take melee/ranged weps. They can also take Melta Bombs and Teleport Homers. So with the exception of points, the Vet Sarge upgrade is a wash. Also, both Scouts and Tac Marines have the same basic wargear, as well as No Fear, Chapter Tactics and Combat Squads.
My vote here is for Scouts. They have the advantage in terms of deployment, objective grabbing and survivability. If we compare 5-man squads, Scouts can be comparable in terms of fire power. Their ability to Outflank is also a significant differentiation. Sure, Tac Squads can arrive by Pod, but all that shiny firepower can't be brought to bear the turn they arrive. The upgrade to Camo cloaks basically cancels the point difference between Scouts and Tac Squads; the improvement in cover save is well worth it, though. Finally, let's not forget that Scouts now have a comparable Ballistic Skill.
I will still have Tac Marine models in my Command Squads, but otherwise will be playing for the first time with an all Scout (Troop choice) list. In the ObjSec meta of 7th, however, it may just work.
The question stands...Tac Squads or Scouts?