Kana F -hp/+atk or -hp/+spd???

Hi. I was lucky enough to get a -hp/+atk and 3 other crappy iv that I merged with the +atk and have her built already with lightning breath+ (+spd) and steady breath and the works... well, I just got lucky and pulled another that is -hp/+spd. I see that this is the preferred iv spread. Should I merge my others into this new one or keep the +atk?