I pondered this issue of a sxs clays gun last year about this time and posted the question to this board at least once. I had narrowed my choices down to basically three guns - an RBL 30, a BSS 30 but with new stock/forend or a Poli. I shot all three at various times and ultimately decided on the Poli and have not been unhappy with this decision. I shoot sporting clays fairly regularly in both competition as well as casually. I currently have a 32 Blaser F3 as my main competition gun but have also shot a Beretta SO4 in competition. With my Poli I can shoot similar scores over the same course and it is great fun to shoot. My gun was imported by Coles and was designed (seemingly) to shoot clays as it has a palm swell, double triggers set at about 3 lbs, Briley thin wall chokes with overbored barrels. The gun has a semi beavertail forend so it is quite trim and the handling dynamics are superb. In my opinion, the RBLs that I have shot feel much less dynamic than this gun but it depends on what you want. This gun is more like a pigeon gun than a heavy grooved swing skeet or trap gun (or some sporting clays guns). Overall, I am extremely pleased with the purchase.
As an alternative, I had looked seriously at building a clays gun on a BSS platform using a 30 barreled action. However, with the initial purchase of the gun (1500), stock work (1500 - 2000), barrel work and chokes (600), etc I quickly was over the purchase price of my lightly used Poli Sporting Extra (side plated model). If you can not find a good deal on a Poli, which I think is an outstanding gun for clays or pigeons, then a home grown BSS clays gun is probably the best alternative for a nice gun around $3K. SKB also made a nice sxs specifically designed for clays but I like my Poli or the BSS route a bit better.
Have a great time with this as sxs are much more fun to shoot competitively than an O/U. In most events sxs are rare and if you shoot it even reasonably well, those that you shoot with will think you have extraordinary skills.
My thoughts
Z