Johnston's is Mongolian fiber woven in Scotland with the garment assembled in Scotland. The description on PRL was woven in Italy and assembled in China. So at a minimum you're paying for unnecessary shipping of the fabric, plus of course the Ralph Lauren label. I would prefer to buy directly from the maker, be it in China or Scotland.
STP prices right now are $120 for a Johnston's cashmere sweater-vest, and $120 - $165 for regular pull-over cashmere sweaters (depending on cut and color) with their dealflyer coupons. These are really good deals, and I would most certainly buy there before purchasing a Ralph Lauren sweater from ebay for $150. They also have good deals now on Johnston's cashmere gloves -- I just got a pair and they are very nice. Also Pantherella socks (highly recommended) -- two-packs for around $15 with the discount.
If any of you want to do that, you can signup in a way that will give them some credit to me by clicking through here. This will give you a $10 coupon as well. Then sign up for their dealflyer email, and follow the link through that to get the discount pricing. It works very well, with the only minor annoyance that STP sends their discount emails ~twice/week.