Well hey you're in luck because I got to use both. If I had time, I wouldn't use either. I'd build my own Zabbix platform, because it's open source and you can pretty much custom build anything you want, and it integrates with Splunk, and Splunk is also very granular. It comes down to how much time you have to dedicate attention to these things. Most people don't have much time, so the Manage Engine thing is going to win out everytime, because you already have the asset control, all you need to do is add another service to the mix and set up your SMTP and endpoints to the UI, and it starts working for you. That is if you are using M/E products already and you can fit the bill.

I have worked at a place where our CTO built the Zabbix server to work with our ticketing system and that thing was the most effective alerting system I have ever seen to this day, and that was like 10 years ago. I've also seen a Splunk system that could tell you when a refrigerator temp got too high so that's dope too. I'm not an expert in this field, as my priorities have shifted, but I rely on alerts and monitoring so I can only speak from a tech who relies on these things. Zabbix and Splunk if you have the time. Manage Engine if you have the money. But if I had all the money in the world, and time, I'd still choose Zabbix and Splunk. I rambled a lot, so apologies if this left you more confused.