I have 2 scopes, a Kowa TSN-821 (24x, 80 mm objective) and a Bushnell (20-60x, 50 mm objective). Kowa is at least 10 times better. While the Bushnell has higher magnification, above 40x or so it gets very dark. So power itself isn't everything.
My recommendations? Get a 80 mm objective with the best optical quality you can afford. Variable power eyepieces will let you get the most magnification that light conditions will allow.
Laphroaig