I ran some tests on the Beta to try to approximate at what points on the eclipse bar Wrath becomes stronger than Starfire and vice versa.
Stats
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11.66% Crit
9.43% Haste
47.08% Mastery
8.18% Multistrike (the data below excludes all multistrike damage)
I took the damage done by Starfire and Wrath and divided them by their cast times to find their effective DPS at several points on the eclipse bar. I recorded five values for each data point and threw the averages (in thousands) into excel and came up with this:
Which shows that:
1) Starfire remains stronger than Wrath until about 1/5th of the way into solar.
2) an empowered Starfire (with SotF) remains stronger than an unempowered Wrath 3/4th of the way into solar.
3) an empowered Wrath (with SotF) remains stronger than an unempowered Starfire 2/3s of the way into lunar.
And without SotF you can see these positions shift toward the center of the eclipse bar:
Tests were done without any of the proc enchants on my weapon for better precision. Keep in mind that damage is calculated when your spell cast finishes and not when you start casting it.