If you only buy one, I'd suggest Lucky Palms. I love the casino items and I've put the run down buildings in the old part of town in a few different worlds I've played in. It's also a fantastic vacation spot if you use Nraas traveler mod. If you didn't know already, LP and the casino will come up for sale on the daily deal from time to time. Are you familiar with this
thread?
Performance is also a primary concern for me. I've had to stop playing games in certain towns when the issues became too much. Pulls me out of the game to have to deal with the technical issues along the way. The two towns that you mentioned actually run rather well on my machine with Twinbrook being a third. It's a pretty common consensus that SV runs better than the other EA towns.
If you do consider the custom content route, I'd recommend Mesa Grande by aaronrogers8i3. No town, EA town or otherwise, has run as well as Mesa Grande has for me. If you're interested, the most recent version is 1.5 and can be found
here. I'm not able to find a thread on the sim3 forums for the 1.5 release but this
thread for the 1.4 release is still good for links to the seasons and ambitions lots he built for it. This town requires CC, I'm using it without any problems and it's easy to install.