Converting from 3 onto EE. 3 signal is adequate at best; It's tolerable for the price of their contracts and perhaps a trade off for some to have unlimited 4g data however don't strong fast signal, or signal at all, when you enter a places like the Trafford Centre or Tescos et al.
Their 4g network, whilst free of charge, is still very weak compared to EE too. I often rarely get it despite being in areas advertised as 'strong signal' (i.e. Manchester city centre), and when I do its only 3mbps faster than 3g. A minor gripe too is that the signal constantly changes from 4g to 3g and vice versa, and for some apps which I may be using which needs a continuous connection, it will always disconnect as it changes between the two masts.
You do realise that 3 and EE basically share the same infrastructure for their 3G networks now via the MBNL network share agreement?
If EE have good 3G in an area, chances are 3 will as well. 4G is different, as it isn't included in the sharing agreement (yet).