The difference is if you get it from a plumber that installs it, if something goes wrong (rare), he is on the hook to fix it. If you get it, and something goes wrong, any labor to fix it is on you (unless Toto will pick it up).
On another aspect, it is really pretty easy to install a toilet (and remove an old one). The hardest thing is that sometimes the nuts on the toilet flange may be rusted. Since you are going to remove it, a hack saw will cut them off - don't worry about any damage to the toilet. New bolts may come with the toilet, and even if they don't, they're cheap and readily available.
If you need/want to replace the shutoff, depending on how it is installed, that may be easy to replace as well, but this assumes that the house shutoff works! If it still leaves a little water running, open a valve before that one in the scheme of things, the water will go out there (usually), rather than the toilet.
The messiest part of this is cleaning up the wax ring under the toilet prior to installing a new one (don't try to reuse the existing one).
Don't overtighten the bolts holding the tank to the base, and the base to the floor. Tighten them up evenly, on both sides. Get the bolts hand tight, then turn them a turn or two on one side, then the other until it is snug. It doesn't have to be as tight as you can get it, only snug. The ones on the base should be tightened until the whole base is flat on the floor and it doesn't move if you sit on it. If the floor is not flat, you may need to shim under it. Some people use a penny or something like that. You may get an idea if the floor is uneven when you take the old one out - did they have to use shims to make it sit without rocking.
Getting rid of the old one is sometimes problematic. I just break them up. A pair of safety glasses and a small sledge will make short work of one - the smaller pieces are easier to carry (although can be sharp - be careful).
Some plumbers don't like to install stuff they don't supply. Same with electricians. Most will do the job on a time basis, but if the product fails and it is not their fault, it is up to you to fix it (i.e., get a replacement).
As noted, toilets rarely arrive defective (broken sometimes, yes).
A non-professional's view.