Which way to merge

Personally I think the +def would work better for her. Even with a boon her res is still quite low.

I'd go for +Res to make her a better mixed tank, but that's just me. It depends on what role you want her to play though, and how she fits in with her team...

+Res. In this meta you get tons of mileage out of mixing your bulk and having her be able to take on as many threats as possible goes a long way,

+res is a support-dependent IV. It represents a commitment to further investment into her res because her res isn't good enough even with a boon and her entire statline is a bit flat which means in general it needs some investment according to what you want her to do.

+def is more straightforward. It's solid with high spd and def; it doesn't require that much more work for her to be good at tanking on her def.