There are a lot of different stump grinding companies out there. I am going to break them down into 3 categories. You have the jack of all trades- but also stump grind, Tree guys that own a grinder, and guys like me, the stump grinding professionals.
I am not going to speak for everyone out there, but I find the quality with a jack of all trades, or a tree guy is usually noticeably lower verses choosing a true stump grinding professional. And I will examine why this is the case.
Sometimes choosing one of the first two results in a lower price on the job, but getting the job done correctly is sacrificed. (I will note however, if you are getting a full tree removed, get a quote from that company for the grinding portion as well. Often if they have a large stump grinder, you can negotiate that cost into the price of the tree removal and get a great deal. Just get the specifics on how deep they will grind, and hold them to that to be sure the job is done correctly.)
Usually a jack of all trades handy man that says he or she stump grinds will either rent a stump grinder, or blow the dust off an old machine they have owned for years. These machines can lack the power to get the job done right and their operators lack the experience necessary to do a job efficiently. They may not have the equipment to ensure proper clean up is done, or the proper insurance. Choosing them may be a cheap option, but know you get what you pay for.
A tree guy that also stump grinds is a step up from the jack of all trades in most cases. I find most of the companies that offer grinding own a tow behind large grinder suited for the largest of stumps. This is great, but many times the operator can be an inexperienced or lack the pride that a true stump grinding professional takes in a job. The other downside is getting these guys to show up. Most tree companies are swamped with tree take downs and only grind when the business slows down. This means it could take weeks to get the company to complete the job.
The grinding professional on the other hand is an expert in his field, has the experience and knowledge to get the job done correctly, owns the equipment necessary to properly clean up after a job, and most importantly has the proper insurance. I know this to be the case in my area because I have chatted up a lot of my stump grinding professionals and found that the majority of the guys that strictly grind stumps take pride in their work, know that their reputation is everything, and possess the proper insurance.
Ultimately who to select is up to you the customer. I always recommend getting bids from multiple companies with positive reviews and comparing the price they give to the quality of their past jobs.