A new upper for your AR will give you the extra punch you're looking for without the cost of a whole new rifle. Of course if this is an excuse to buy another rifle, by all means, don't let me stand in the way!
A 6.8 or 6.5 or 243WSSM or x39 or (insert any of 1000 caliber options here) are all more powerful than .223 in the same AR-15 format you like.