The bus is certainly an option, although I doubt you have any flexibility in terms of which places you want to visit. The price is hard to beat, so if you are ok with the "fixed menu" format and are on a budget, then perhaps it is indeed one of the best options.

For the more serious wine aficionados, hiring a private car and driver is the best way to go, even though the economics involved are quite different. However, with a private tour you will get the following advantages, just to name a few.....

a) tailor your visits to exactly where you want to go
b) skip the standard visit to see yet another stainless steel tank and go straight to what you want to do - ie. taste wine
c) get a chance to do barrel tastings
d) at some places, you may get a little time with the Winemaker or else someone from his/her team
e) request to taste the high-end wines (many times as no additional cost)
f) have access to Bodegas that are not reached by the Group Tour Operators

The bus tour option is great to have and in no way do I see it as a bad option, it is relevant however to note that for many visitors that are more serious about wine and that can do their math in US$ or another hard currency, a private tour is a far better and still reasonably priced option... especially if your party is of 2/3 people.

As Ive gone on record before, Miguel Sanz is one of the best private guides in the business.... and I would not hesitate to contact him for a quote if you are planning on visiting Mendoza.