Without the Business Amex Platinum card, you can redeem for revenue flights at 1 cent per point, similar to the CSR's 1.5 cents per point. So the 500 points earned on the Amex Platinum are worth at least $5 ($10 if you have the Business Platinum Amex), while the 300 points earned on the CSR are worth at least $4.50.

If you use the points for other things, like transferring to partner frequent flyer programs, there is plenty of variance of opinion as to what the points are worth, but unless you really want United, Southwest, or Korean miles, 500 Amex points are probably worth more than 300 Chase points.