afaik the only advantage would be continuous contracts. since zen is free and nt7 will address continuous contracts i would probably just try and hold out. supposedly nt7 will be out before the end of the year.
unless of course you are trading something other than futures...
i've used esignal before and the feed was fine no complaints but all their api software crap just added another layer of complexity and another layer of troubleshooting if things went wrong. i never had any of these problems with zen fire afterall ninja was written for zen.
"Let us be thankful for the fools. But for them the rest of us could not succeed." - Mark Twain
If you use ZF you should not enable historical data because that data is not real it is random generated (changes) data and cant be considered realistic for testing purposes. People always test on this and wonder why they get different results all the time. The downside is you have to leave the NT on to collect ZF live data as opposed to Esignal which loads it from the server. Esignal if you get disconnected from the broker your database wont get messed up because Esignal bars are pre made on Esignal server. As opposed to ZF which sends only the ticker streaming data and NT itself makes the bars which if you get disconnected can mess up the database. Also the continuous contract however some people have figured out how to merge ZF I have never been able to. Once when I paid for Esignal I noticed discrepancies between the feed and the broker price feed. However this actually seemed to work in my favor giving less false signals strangely enough as Esignal wasn't as spiky. I believe Esignal takes its data from multiple global points and averages it whereis ZF comes straight from the CME ticker which is wild and fast. But it costs alot and if you don't have alot of money you have lost money before you even start trading.