for side imaging anything less than 7" is a waste of money. all 4 of the major brands will show fish returns. some will have better cover returns and some will have better fish returns. and then there is budget, how much are you willing to spend?
side imaging only will not make you a better fisherman. a combination of side imaging used in conjunction with 2d sonar and down imaging WILL help you find more fish.
with 2d on split freqs i can cover a lot of water at 20mph and mark those fish returns. then go back with down scan or side imaging and get a better location and actually see the cover/structure they are holding.
with side imaging i am scanning shallow water that i cant run with 2d on effectively, this is where side imaging comes in handy.
on lake ledges i use 2d and down imaging more than anything. i run back and forth across the ledges/channel drops to find and mark fish. i may do this for anfew hundred yards before i even pick up a rod.
as far as specific units and side imaging, in my opinion, humminbird is hands down on cover detail. to my eyes, lowrance and garmin are about the same except garmins range is greater and with a GT30 transducer its probably better on detail. ive never seen a ray marine but almost bought one. keep in mind ive only seen first hand and used the upper mid range units in the last 2 years. you could PM Trigger and ask him for a comparison between ray marine and humminbird upper end units.
as far as seeing what fish look like on side imaging, check out this video. he can show it better than i can explain it.
https://youtu.be/3lu_lQtLqpE
then you have the mapping issue. most now include some sort of base map but the better maps and features you have to buy a map card. garmin comes with really good mapping and also sells an upgraded map package, BUT with garmins you cant use navionics or other mapping software. garmin bought navionics so that may change in the near future. humminbird lakemaster maps are good too.