I would say in this instance no. If you build a succesful site on a .net, you are handing the owner of the .com a chance to develop something along similar lines and hijack your traffic, or at best, market their site to compete directly with yours.
That said, this is probably more relevent with a brandable name. If its something like “onlinecasino.uk.net” as in my sig, its less relevant as the name is not one that will encourage loyalty through its lack of obvious branding effectiveness. If however its something more easily memorable like pokerstars.net, then without the .com, i would be more inclined to sell it on and buy a memorable “.com”.
If the name is just for SERPs purposes, then it matters less if its .com or .net, but you are still opening the door for a potential competitor to use your hard work to steal your traffic, especially as these two extensions will be competing for the same regional SERPS. Aside from which, the days when keyword-stuffed domain names being so important are definately starting to recede IMHO.
Cheers
Simmo!