Confused on what AC Unit to purchase and install?
The heat pump can move heat indoors to outdoors ("air condition") as well as from outdoors to indoors (heating).  The cooling only unit can only air condition.

If the house already has heating or you rarely need heat, you can save some money by getting a cooling-only unit, and maybe supplement that with portable electric heaters as needed.

As to brand comparisons, I strongly recommend you review the Consumer Reports reader reliability report of air conditioners and heatpumps:  that can be done online by paying a fee, or visiting a library.

As for some installers carefully measuring and the others not, the latter could be fine if your dwelling is not especially unique:  an experienced installer has run the numbers so many times that they know what the result is.  That experience is helped by there being a coarse choice of capacities:  2 ton, 2.5, 3, 3.5, 4, 5, 6, 7.5, 10, 12.5, etc.

On the other hand, not measuring could be a sign of an unknowledgeable installer.  Do they also propose a 3.5 ton unit?

As far as questions to ask, I like to elicit being assured that if anything goes wrong for X years, they will fix it for free.  If they can say so confidently, that is a very good sign that they don't think anything will go wrong.

It might be useful to ask an installer why they choose 3.5 tons and not 3 or 4.  There is a certain degree of assumption about worst case and average case conditions.  If the a/c can't quite keep up on the 5 hottest days that is one thing, but if it can't keep up on 30 of them, that is quite a different matter.  Likewise, an oversized system can chill the air so much that it doesn't have to run much, making the air stagnant and possibly muggy.  Questions asking about how they balance those factors are good for you to understand and for the installer to reflect upon.