Mo, I measured my XXSP Halogen tank for you to help you decide.
Top of strap to bottom of armhole: 7.25"
Bottom of armhole to bottom of tank: 16.5"
Bottom center of neckline to bottom of tank: 17.25"
Bust (base of armhole to base of armhole): 28"
A note about the bust...I am about 31.5" in the bust, and the 28" here does not seem too small to me at all. The material is so soft and stretchy that it is not a problem. In fact, I wouldn't have minded the tank being even smaller in the bust for me.
As to the straps, I don't know how successful you would be getting them to hide bra straps. At their smallest point, they are only .5" wide.
If you are wanting a soft tank, I would definitely say give the Halogen a try. It is not constricting in any way.