Well, firstly the Sig 550 is prohibited. The Classic Green rifles that we have available to us, although externally identical to the Sig 550, differ internally enough to not be considered a variant.
As for the rifles, each has pros and cons.
The Swiss Arms is the heavier of the two rifles. It is made of steel stampings, reinforced by machined steel parts as required. If you get the Classic Green model with iron sights, an additional rail must be purchased in order to mount optics. Conversely, there are flat top models available with an integral picatinny rail, but no iron sights.
The SL8 is a (
fiber reinforced) polymer (
Nylon 66) receiver reinforced with steel. It is lighter than the Swiss. (
the SL8-4 has a short rail no sights, the SL8-5 has a long rail with iron(plastic) sights. Not the best sights as the are a bit large.) If you wish to turn it into a G36 look alike, you will be required to purchase conversion parts and the correct optics/red dot carry handle. This will bring the cost over that of the Swiss.
(there are other options for stock customizing from H.E.R.A Arms. There are also rail and accessory options available from other companied at many price points. See the link I posted below)
Factory magazines for both of them are expensive, but there are knock off Swiss Arms magazines that function just as well as the originals, and for less than half the price.
The SL8 can have a magwell conversion done so that it accepts AR-15 magazines. This cannot be done for the Swiss; however, NEA makes a conversion lower that takes both AR-15 magazines and AR-15 stocks.
(there are also Spear magazines and now magpul magazines available for the SL8 that do not require a new magwel)
Also bear in mind that the Swiss Arms is about $1000 (after tax) more than the SL8. They are both very accurate rifles. A consistent 1.5 MOA should be expected from both rifles, using decent ammunition.
For ALL things SL8 go to this link:http://www.canadiangunnutz.com/forum/showthread.php?t=270868
And for all things Swiss Arms: http://www.canadiangunnutz.com/forum/showthread.php?t=513834