I think both options will be available for a long time. Both are open source, so you can host yourself, or pay someone to do it for you.
I use both for different things. I have been using OEM for a decade or more. I started when they were free and moved a bunch of stuff over to them when SmartEnergyGroups (from Australia) started charging. I probably would have stayed at SEG, if they had had reasonable pricing and easy billing. But, that was not the case. OEM started charging €1/feed/year with easy billing (but credits, which are deducted by usage). I think I still have about 50 active feeds. I really should get rid of some more of them, since I don’t really use some of them. They have historical data and are inexpensive enough that I have been lazy.
The better question to ask is which one provides the information you and your customers need in the easiest to understand way. InfluxDB integrates well with Grafana, which makes it possible to create some beautiful charts/interfaces. It might be possible to do that with OEM, but I don’t believe it is built in.
Of course, with that great capability comes the responsibility of creating the interface. I like InfluxDB/Grafana because I can aggregate the data from multiple IotaWatt units.