I'd call R&L CS & see if there's any local stores carrying the brush so you can see it firsthand. Also, ask them if you order online & use it, can you return it & if you don't use it, is there a return restocking fee.
It's $25 + shipping & if you have to return it, you'll be out shipping both ways. For < $14 more, you're better off buying the MAC during the sale because you know MAC's quality, the great reviews the 138's been getting, & if you need to, you can return it w/o any problems. To save < $14, I wouldn't order a brush I've never seen which from the linked thread has only several reviews; one of them negative.
Although the 138 hasn't been stocked in my MAC stores, I've gotten to try out the SAs 138s on a couple of occasions & it is a super nice brush that I'm ordering during the sale.