Replacing A/C-Need your opinions

[deltadart]

Bought this house 2 years ago and the furnace/central air was installed in 1994. It's a Heil high eff. unit. We have no problem heating the house. Gas bill is very low for this area(Cedar Rapids Iowa). My problem is with the central air. It just doesn't do a good job. It struggles to lower the temps. Had the unit all checked out and was told it's working the way it should. There running some good rebates in this area and I was wondering if it would be a smart move just to have the cental air unit replaced with the latest high eff. unit or it would be smarter to replace both heating and air at the same time. Thanks!