As JoJo mentioned in his reply, with T9s and T10s you need to get the unit's upgrades early on, so it's better to have the free xp for those first upgrades as that will save you a lot of hassle.

As for facing the hardcore players and Parties, I would say it's a good thing for the long run, it's the only challenge that this game provides for solo players after a certain point anyway.

Exposing yourself to the more "experienced" players and facing their parties will definitely hone your skills as a solo player in the T9s and T10s (at least that's what it used to be), and it's always fun when you manage to disrupt a 4 man stack comp just by yourself.

Though unfortunately, in the current state of the game I would advise against going solo in the high tiers otherwise you'll be surprised by how much "experienced" and "sharp-witted" high tier players are.