Agree with LAX's comment. Look at the aircraft. If it is an A319, or even an A320, upgrading will be hard with an e500. However, if it is a 757, 777, or 767, it should be okay if you were a 1k. But if you're premier, even the e500 may be hard.

If you don't want to chance it, use miles. That puts you ahead of all the e500's who are waiting (even those with higher status than you). But if you wouldn't mind sitting in economy and want to save those miles, then go for the e500.