I would recommend either the Nordic Components tube/barrel clamp (w/accessory rail) or the Insight Technologies shotgun adapter (tube mounted). I have used both of these on my Benelli without issue. Unfortunately, these accessory clamps require the use of a mag tube extension.
In terms of the rail question, Picatinny seems fairly standard (vs. Weaver), from what I have seen. I have heard that Picatinny and/or Weaver based accessories are not necessarily compatible, but I've found this is not always the case. Either way, I would stick with Picatinny as it seems to be more consistent vs. Weaver.
But, having said this, it basically boils down to what light you plan on using and then determining whether or not an appropriate mount is available for your particular gun. Keep in mind that not all lights necessarily require the use of a "rail" system. Just something to think about.
BTW- If needed, I have such accessories for sale (including the previously mentioned Nordic Components clamp w/rail). PM me if interested.
