It’s down to personal preference as the lower grade horses have lower stats at tier 3 and 4 meaning that if you want to diamond skill them it will require more costs and time. There are some lovely horses at all levels though.
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You can find my full up to date horse list at
https://hhwa.fandom.com/wiki/User:Solara184 I am far more active on the wiki than on this forum.
You can also post on my wall there (you can do so without an account) to request a specific horse, I play on iOS and have my own club so we can always coordinate a club invite through the wiki chat to get on one another’s friends lists as I am not on Facebook.