|Left: Sigma E25, Right: Mac 217|
As I used the E25 I discovered that it's slightly different from the Mac 217 - not worse, not better, just different. The E25 is a lot flatter in shape than the 217, which is more oval all-around. This has its pros and cons. Pro: you can use it to pick up more shadow and place it all over the lid more efficiently. Con: it loses the ability to be as precise as the 217. Also, the E25 is denser than the 217, so you have to work a little harder to soften those edges.
The main reason I would advise picking up a Sigma E25 is the price point - $18, versus $25 at Mac. Sigma is a really reliable makeup brush brand, and they're my second-favorite right next to Mac. Whenever I feel like I need a certain brush but don't want to dent the wallet too much, I turn to Sigma! The brushes are sturdy and well-made. As a piece of advice, I would recommend the white bristled brushes over the black bristles, as the black bristled ones do tend to shed a little bit, but generally Sigma is a great mid-end brand for makeup brushes. Though more expensive than Elf they're higher in quality, but lower in price and comparable in quality to Mac.
Overall the E25 is a really nice addition to my brush collection. After all, you can never have too many eyeshadow brushes!