There will be locations where O2 gives better coverage than EE, and locations where EE is the better option . On a national basis, I would expect an EE phone to generally outperform an O2 one, but it won't be the case everywhere.
The online coverage webtool provides a basic prediction of coverage in any given location - https://ee.co.uk/help/mobile-coverage-checker - does this broadly match what you experience? I would also always recommend testing the coverage of any prospective new network with a PAYG phone before making a definite switch.
WiFi-calling is provided as an indoor coverage mitigation and works very well - you mention "very poor signal outside", is this preventing you from making or receiving calls? I'm wondering if this is just the "only one bar" perception that many users have, or if it's a genuine inability to use your phone for calls, texts and/or data.
The answer to whether you can leave EE depends significantly on where, when & how you purchased your EE service(s).
