Riley and Garrick soulstones are in "Epic Arena", which allow more points than "Arena"...for the rest, it depends by the meta of your server, by your goals and by your personal preferences.
I dun like so much Garrick, but i love Riley, so i have the wolf child 7*, and Garrick 5* (thanks to gold chest 10x).
It's better to build 1 solid team to compete in Arena, if this is your primary choise, adding 1 healer (Jasmine or Flora) to make CoF an easy task...and obviously other heroes you like/you can level up (by farming SS, HoL, CoF, Arena, Guild Store).
The nice thing about Soul Hunters is that you can opt for many different solutions, that can take you to the goal.
What can i tell you is that when you'll reach 90, with a lot of heroes max 5* and without at least a team of 6*, you'll be cannon fodder.