I personally have never had good luck with whip stitch. It always, for me anyway, looks lumpy and uneven.
Even if I fudge a bit with mattress stitch, though, it always looks good.
I'm not much of a sewing person, so if you feel comfortable with whip stitch, go for it. It's your sweater.
I'd suggest trying the mattress. It comes out easily if you need to fix something or you just don't like the way it looks.