Elemental summons order is a follow : Red - Blue - Green - Yellow - Purple - Red… Switch every two days.
The difference between elemental and epic summon, is that epic is from all the 3* 4* and 5* heroes, and elemental is only one color. A lot of people tend to go with elemental for a better chance at the hero they want currently, or they may think might improve there team the best.
It is also said that elemental summons have a better chance at getting better rewards. The precise number of those chance increase are not public and i personally don’t remember reading anything about it. Be prepared to spend 300 gems and be disappointing with 3* hero, it happens a lot.
Now the hero of the month (hel in june, athena in july) seem to have a bonus chance of 5-10% to be gained with any epic or elemental summon.
I use the 3rd team option to bring specific hero higher up on my hero list, (those in teams are always at the top), and also you could have a different team for attacking raids, defending raids, titans, map quests. Its just a convenience option more then anything. I personally would enjoy 5 teams.