start small
Unless you know you have the workload to support both a small and large stump grinder, start with a small self-propelled. With a smaller unit, you can do any size stump. Granted, the larger ones will take a while but, at least you won't have to pass up those stumps that you otherwise couldn't get at with a tow behind unit. I ground hundreds of stumps ranging from 24-60" with a vermeer 252 and it worked fine. Yes, the 60" ones were a pain because, with a small unit, you have to reset up a few times; however, it got the job done.
Once you can justify purchasing a bigger unit, there's no need to start out with a $40k unit when there are plenty of good used units on the market for $5-10k. I bought an older vermeer 665A for less than $5000 and easily paid for it in less than one season working only part-time. It grinds out a 60" stump in about 30-40 minutes and is very easy to maintain.
Yeh, I'd like a new machine with all the frills but, I can't justify the payment working part-time for only 7 months of the year in a colder environment. For me, a smaller self-propelled unit accompanied by an older, yet reliable, tow-behind is a great combination.
If you buy both units up front, you can shop around and probably get set up with two used yet, good units for 10-15k. You may have to deal with a few quirks here and there but, at least your overhead will be low starting out until you build your business clientel. Then, when you're comfortable with your business, you can upgrade to a newer unit.
Good luck.