Which one of these two will give me better handling, assume that I would be using either a high quality frame brace or strut bar

I have an xB with 18's and Tein H-Techs

I built my own front subframe brace, it's stiffer than anything on the market commercially, I noticed only a slight increase in cornering stability at the limit of traction. If you don't drive that agressively you won't notice anything but it may stop some body flex that causes squeeks as time goes on. You can see my brace at www.e21motorsports.com/kendanielson , go to the bottom of the xA page for pics. Compare the design and strength to any on the market. I have a jig for it, PM or email me if you want one.

I don't have a strut brace on my xA, the strut towers are very close to the firewall so I don't think a strut brace will help much either. I'll get to building one in the future though.

Chassis stiffness is ALWAYS good. Install them both but don't expect any huge revelations.

The real weak part of handling on these cars is the rear torsion beam pivot bushings but I don't know about any urethane replacements out yet.
ken