Silk is pretty forgiving of a heavy spindle, given its long staple length, so the 1.6 oz might work; but it depends also on how thick you want to spin, how slip-y the silk is -- it might draft its self right out of your hands with a heavier spindle. I learned to spin silk on a 1 oz. Lollipop spindle (small toy wheel spindle, basically, though _very_ pretty!). I've spun laceweight silk on 1.5 ounce spindles, but personally tend to stick to closer to an ounce since my wrists tend to ache with heavier spindles (RSI -- recurring tendonitis).
On the other end of the spectrum, the 0.4 ounce spindle will probably only spin cobweb and laceweight -- I tend to reach for nothing under a 0.9 ounce for plying, even for my laceweights; a sub-half ounce spindle tends to stop too quickly with thicker yarns (singles or plying).
What thickness are you looking to spin, or are you simply looking for a good all-around silk spindle? If it's the latter, I'd recommend looking for a spindle with mass near the shaft -- a dense wood shaft helps concentrate mass there too (purpleheart, ebony, cocobolo as shaft woods all help here). This gives you a
fast spin, which helps get the twist quickly into the silk before its own lack-of-scales-and-crimp helps it slip apart.
The micro-second lesson on spindle physics this is hinting at, is, rim-weight (outer edge of whorl) == long spinning time; shaft-weight (inner edge of whorl/dense wood shaft) == fast spinning time. For a great combination of both of these, a Bosworth spindle has both bulk in the whorl at the shaft and at the outer edge of the rim (AD since I do carry these :-) )
