Avoid the crap
Some advice before you go selecting one of these packages because there is a lot of crap out there...
Find out if the vendor actually has some clients who are using it and then get in contact with them. Find out what they are using it for and see if there are any similarities between their operation and yours.
If you are going to be the guinea pig for someone else's software then insist on a grace period in which you can opt out of it with no obligation if turns out to be crap.
If your ops is going to be 24/7 then make sure their support is also, put it in the contract and make sure they stick to it. The cowboys will scrimp on service.
You'd be surprised how many of these outfits cut and run as soon as you turn the screws.
If they take days to answer simple questions then beware of that also.
No one says that you have to go with some of the big brand names but don't be afraid to approach them either. Some of them understand that the little guy still contributes to their bottom line and may be willing to tailor their prices to match.