I use a Penn SS6500 for saltwater spinning, and I agree: they are indestructible, excellent value-for-money reels. But they are also heavy for their line class, too: there's a lot of iron in there.
One alternative you might consider is the Daiwa SS Tournament Series 2600 (or maybe one size smaller, the SS1300 if you're certain you only need 100 yards of heavy line). The SS2600 is the one I use for light saltwater and spinning for kings. First, it has an excellent drag -- lots of surface area and smooth. (After a few days fishing for mackerel and trevally, you appreciate that few reels have drags worth a hoot. The Penn's and this particular Daiwa measure up.) Second, either the 1300 or 2600 has the line capacity you need: both handle 210 yards of 10lb (I can't read the other marks on the spool as they've worn off, but I get around 130 yards of Maxima 20lb Green or 225 yards of 20lb Fireline). Third, it has a big diameter spool so it casts far. Four, it has a graphite body so it's light and easy to cast all day even if you opt for the bigger SS2600. Five, it has 3 ball bearings and a sure-close bail. Six, I've put the reel through hell and it's held up extremely well.
Price is the only downside. The SS2600 is $130 (plus about $20 for an extra spool). The SS1300 is $120 (both from Bass Pro's Master 1999 catalog, p. 39). This is $40-$50 more than the comparable Penn ($80 + $14 for spare spool). For my money, I'd pay the extra and save on the weight.