Here's the list for Shimano road components and for MTB components:
Shimano Road Component Hierarchy | Derailleur | Crankset : ChooseMyBicycle.com
Shimano MTB Component Hierarchy | Derailleur | Crankset : ChooseMyBicycle.com
I have trikes with both Tiagara and Shimano 105. I like the 105 much better than Tiagara even though there is only one step between them. The 105 is a very nice mid-level component and in my experience will work well and last a very long time. I very rarely had to make any adjustments on the 105 components. The finish on them is first rate.
If you buy a used bike that is the proper frame size and it weighs less than 30 pounds you will rarely go wrong. The crap bikes are invariably heavy, usually much heavier than 30 pounds. It takes decent components and a decent frame to get the weight down below that point. The fact that the bike shop is steering you toward an inexpensive bike with front suspension tells me they are more interested in making a sale than in getting you on the right bike. Front suspensions add weight to the bike and they just aren't necessary for the type of riding you describe. They are a great selling point for the customer who knows little about what makes a decent bike. Try another bike shop.