AT my current day care (SW Bris) 3 Jessicas; 2 staff, 1 [adorable!] child. No Ellas
At last day care (Inner East Melb) 1 Jessica, 1 Ella
At the one before that (Inner South Melb) 1 Jessica, 4 Ellas.

So, even though the stats say they're super-common, I actually think you're pretty safe with either.

At one place that had 2 Ellas, they were referred to as Ella-Grace and Ella-Rose, their middle names, rather than "Ella A" and "Ella B" (which I HATE!)