there aren't really advantages/disadvantages for any particular system. it's more a case of if a particular script will do what you want it to and what the support is like.

Like bluedreamer said, it really depends on your needs and how much you're willing to spend.

ClientExec is cheap and has lots of features, but not as many as ModernBill. I don't know how good ModernBill's support team are, but ClientExec's technical support reps are nothing short of amazing.

Try a demo of both of them, and see which one has what you need...

You should evaluate MB (Modernbill) as well.

You should evaluate MB (Modernbill) as well.

They're coming out with a new version soon aren't they?

just adding some meat to the bones here's some questions to ask yourself about each script...

1. does the software do what you want it to?
2. can you understand how to use it?
3. can you extend the features easily at a later date?
4. does the ownership cost fall into your budget?
5. is it customisable, blend into your own site
6. is it user friendly, will your customers understand it?
7. how good is the company support if you need it?

... if you find one that meets all your requirements that is the one to go for