I generally get between 50-120Mbit on Voxi (Vodafone), when I was on Three I got something like between 1-100Mbit, but mostly it tended to hang around 20.
The Vodafone mast is very close by, the Three is more obstructed, though I tried from outside my GP surgery where I can see the mast itself so perfect line of sight, it still sucked.
I don't think there is any magic sauce to this, its just down to which network is the most congested in your area. Three marketed their Unlimited service really hard whereas Voxi is less well known. I also think Vodafone mast locations are much more strategic so spread the load much better than Three in my area.