Depends on your party make-up and the specific campaign. If you already have tons of melee damage, then adding another extra body won't make much of a difference. If, however, your group is lacking in the damage department, that extra body can be crucial to survival. If it's a wilderness campaign with no druids or rangers in sight, then feral speech suddenly becomes a better option.
If your DM allows the retraining rules, I like to take split-slot at 5th level. To me, having the extra lower level slots is better below 10th level, then you can retrain split slot for a more powerful discovery later.
The alternative to balanced summoning is just summoning 1d3 creatures of the same type. You don't need an arcane discovery for that.
If you use blasting spells at all then creative destruction is a useful arcane discovery. If you like battlefield control then fleeting spell may be a useful metamagic feat. If you're having trouble getting higher level spells due to the lack of downtime then persistent spell or empower spell can make lower level spells useful in higher level slots.