I think (from currently looking at them for the same reason as you) the Essex flange is slightly less restrictive in the amount of water it will suck.
Surrey flange goes in the top of the cylinder, Essex can go in at the side, both meant to reduce amount of air sucked in with the water.
I think the Essex flange needs slightly more care with positioning as the slightly curved tube needs to curve away from the side of the tank, or it can suck some air that forms as bubbles against tank edge.
With a 1.5bar pump either should be fine, some places I've looked at say the one to use is determined by age and condition of the tank, as this may affect removal of pipe at top for fitting a surrey, (old tank = seized or corroded pipe) and knack erred tank = possible cylinder tank damage.
I've also seen some sites say a t-joint from the pipe at the top of the cylinder without a flange is ok........so on that aspect I'd say wait for a more experienced opinion than mine

Of course, I could be wrong about the whole thing
