All I can say is - try em. If you like em, keep em, if not, swap it.<br /><br />It's personal preference.<br /><br />Riser bars are "in" right now and it seems most new bikes comes with them.
Whether you choose-and-use a FLAT bar or a RISER bar depends on the relative height of the bar to your saddle that you prefer since the 'sweep' should really be the same on all the bars regardless of 'rise' ...
If you thought your old bike fit you & the type of riding/terrain you did, then adjust the grip height on your new bike, accordingly, using either a bar with an appropriate rise OR more/less headtube spacers.
Hopefully, you remember the height differential on your old bike (if you don't have it) between the top of the saddle & your grips ... or, if you still have your old bike, just match the difference on the new bike with your old bike's setup.
For cosmetic (!) reasons, MY preference is for a FLAT bar, but I've got a couple of different bars with different rises in case I ever conclude that the (relatively) steeper descents would feel more comfortable than using the FLAT bar.
As MtnBikerChk said, you've got them, so you might as well try them ... you might like the (potentially higher?) grip position better!