Keep both. If you’re planning on fully ascending your Grimm all the way to 4/70, you’re fine. Keep going. If you’re only going to 3/60, then just stick to feeding him 10 1 star blues at a time for a while, and you’ll also be fine.
Each stage takes more XP than the stage before it, so you’re not actually as close to 3/60 as you probably think.
Here’s the basic leveling strategy I use when leveling 4 stars to 3/60:
I split the XP required to get to 3/60 into seven equal pieces. You need to trigger the special skill upgrade 6 times (since you get 2 skill upgrades each time you ascend), so this gives you a buffer of almost 17%.
Special Skill Level: Tier/Level
3: 2/1 (ascension)
5: 3/1 (ascension)
If I am below the threshold (i.e., at special skill 3 at 2/14), I feed the hero high XP feeders. If I am above the threshold (i.e., at special skill 2 at 2/14, like you), I feed the hero 1 stars of the same color in groups of 10 until I get back on track.
There’s so much XP required to go from 3/60 to 4/70 that you don’t need to worry at all if you’re going that far.