They're both good options, it's just a matter of preference. Are you looking to build an entire army or just a platoon? They're all nice figs, but being metal, there isn't much variation available.
Collecting Citadel miniatures has pics of all the bare metal figs. Otherwise, the GW pics are actually some of the best you can get, but going to Dakka and putting either of their titles into the gallery search and you'll get a ton of results. Below I've put a link to the GW range currently avaialble and the Collecting Citadel Minaitures pages which show the entire range, some of which are not currently available (probably available on ebay).
Steel Legion: Very much like WW2 German Paratroops. Fairly clean lines, typical of when they were released (late 90's, IIRC). As seen here:
GW
link
CCM
link
Vostroyans: Sort of like Tsarist troops. They are much more ornamental as they are more recent figures and that's the way GW has been going. As seen here:
GW
link
CCM
link
Some other things to consider based on the currenlty available ranges:
-Heavy Weapons, they both have access to a lascannon and heavy bolter, though not necessarily in the squad pack, with SL also having the Missile launcher. Also, as the HB and Lascannon are on tripods or bipods, they can all be fairly easily converted to autocannon if you so desire.
-Special Weapons, both have a plasma gun, but the Vostroyans also have a flamer.
-Variety.The Vostroyans have more sculpts and more variety including medic, coms, Standard bearer, etc.
I like them both, but Vostroyan wins the day for me. I just like the style. It's over-the-top, but it works for me. I don't know when I'll get around to them, but when they appear at bargain prices at the FLGS auction, I snap them up. I've got about 20 now and I'm not sure whether to go for a full platoon or use them to make some ornate commande squads. Note though that my decision for Vostroyans may be colored by the fact that I've already got an IG army of Cadians with helmets and gas-masks that makes them look very steel-legion'ish.