LaVillita is a proven consistent attractor and it's at close-out with single family homes w/Darling and K. Hov. Portrait still has some new town homes for sale. When it finishes, you won't be competing against any of the builders. Positano is newer and it'll take a while for North American Properties to sell out. However, if you're planning on holding it for at least 5 years, they'll be sold out by then, too.
Positano also has a cabana w/a lounge and fitness center, pool/spa, sand volleyball, and BBQ grills. La Villita has the lake (however you can not swim, fish or do anything in it), greenbelt and then a pool and park coming. Both of these which would be very attractive to a young, active, social type tenant who tends to be the demographic moving to the area. The only amenity that Riverside will have is greenbelts, walking trails and possibly a park. In the Lakes of Las Colinas, there will be water features, but these are going to be north of $700k single family homes. Not your typical rental area.
La Villita has been around longer, so it has more ability to provide an appreciation factor. Positano doesn't really have that yet. Check out my blog below. No one has a crystal ball and can guarantee you a certain appreciation factor. However, based on past performance, the new communities coming to the area and the fact that Irving is supposed to grow to 5 million people by 2011 w/an average income of $109k p/year, nice appreciation looks like a good possibility.
We also just helped a client save more than $45k on his brand new town home in the Valley Ranch area. We work this area all the time and would be happy to provide any further info. you'd like.