Fishing - mono a mono

Is there really any difference in mono as far as casting is concerned. If so
what is the best to buy and why

Yea there is. There are a variety of differences. Diameters, stretch, abrasion
resistance, limpness, spring etc. No one line suits all reels, or anglers. For
a multiplier I personally prefer a limp line whereas on a fixed spool a springy
line can aid casting. A lower diameter aids distance and an abrasion resistant
line helps in snaggy areas. So it's down to reel type, area to be fished and
personal preference. A good all round line at sensible money is the Daiwa
Sensor, especially now they do it in clear. I hope this is of some use to you,
cheers, Norman.