The base custom version of kACARS costs 40$. I would not suggest smartCARS as it depends on its developer's server(s). If, for some reason, he decides to close his server which hosts the chat system, you'll have a lot of problems. You know nothing about him, so you will not have the chance to get your money back. Also, i would not cooperate with someone who tries to advertise his program via sending emails via your phpvms's contact form in order to make you buy its acars system. My opinion is that the key of succees is offering good and reliable services, not trying to find new customers via disturbing them.
Personally, i do prefer kACARS as me and Jeffrey have worked a lot in order to built a very good and well designed acars system which is not like the base custom version of it. Moreover, his support is great and you have the chance the test the program before buying it via using the kACARS_Free. More information can be found here (of course the price it higher...). Of course there are a lot of other programms which are realy very good! That's all for now...