It's pretty much a toss-up. Both might drop in value a bit in the second half. The risk with Beckett is the high number of pitches he's throwing, while Ishii might be more hittable as the league becomes familiar with him. If this will clinch a good deal for you, by all means add the sp swap.
Personally, I'd be happy with either one, perhaps with a very slight edge to Beckett, in spite of Florida's bullpen. Similar risks, with a slightly higher upside. If it's a keeper league, though, you naturally really, really want Beckett.