From what I've gleaned so far:
Scandi lines generally have a longer front taper and are best used for lighter flies and without tips.
Skagit heads are short and heavy and work well with tips and heavier flies.
A nice compromise between these two is Airflo's Rage Compact. It's got a longer taper like the scandi line but has mass like a skagit.
Match your grain weight to the rod without factoring in tips.
Now where you have a decision to make is do you want heads or do you want a line with integrated running line. This really depends on how you fish. Heads give you versatility to swap out based on how and what you're fishing, but it's a bit more of an upfront investment. Cheaper though than a second spool and easier to port around.
A fully integrated line locks you in to one line unless you're going to have another spool. The Chucker line is certainly worth consideration, you should read though the discussion on it if you've not already done so.
Rio Switch Chucker Impressions
FWIW, I think your shop is right about the Ambush TTT line. Nothing against it, but if you're going to go with a head and integrated running line, Chucker is a better choice.
What kind of flies will you be using?