This depends on so many different factors including signal strength where you live/work, cost of the tariff for your chosen device/usage, the features the network can offer you etc.
In my situation EE suits me better right now. Reasons being:
I get a good 4G signal in most places.
I like the way they manage data using add-ons so if you run out of data or if you visit another country you have to buy an add-on to get data i.e no surprise bills. Vodafone would start charging you extra per mb.
I have an iPhone so features like Wi-fi Calling and Visual Voicemail are available to me on EE but not currently on Vodafone.
For an extra £4 I get a second SIM on my account and can share my data allowance on my iPad.
It's all down to personal preference though, you may get a better Vodafone signal over EE where you live. If the features above don't matter to you and you can get 20% discount with Vodafone then this may be a much better option for you.