Check the advice section on Starwood for most of the information you are looking for about Starwood and resort locations.
If you buy a 2 bedroom lock-off, you can stay in the 1 bedroom for a week and the studio for a week, so you can get two weeks. You can also exchange within the Starwood Vacation Network at 8 months out using your Staroptions (assuming that you buy from the developer OR buy resale at a mandatory SVN resort). So for example, if you bought a 2 bedroom lock-off with 148,100 Staroptions, you could use 81,000 Staroptions for a 1 bedroom and 67,100 for a studio. At some locations, you can get a 2-bedroom for those Staroptions, so in theory you could stay two weeks in 2-bedroom villas...it just depends on the season and location. Just be aware that you are competing with all the other SVN members at 8 months out, so if you are looking for a high demand week, you might not get what you want.
HGVC is a great system. You should know because you already own there. I haven't heard about anyone not getting the week they want with HGVC. We like the locations of Starwood resorts for our family right now. I was thinking about buying HGVC, but haven't taken the plunge yet.
Good luck! They are both great systems, really. Have fun!