Personally I'd steer clear of both. DesignCrowd is a crowdsourcing site, meaning you'll be working with no guarantee of payment, as will many other designers. FlipaJob is another example of a site that devalues what we do, people offering logos for £30 etc (though if you post a job for £100 and it's actually worth £100 then so be it).
I know a few people have had some success with PeoplePerHour.com for small jobs that bring a bit of extra money in between better paying jobs. Be prepared to be undercut by a lot though. Like most online job boards, people will repeatedly undercut until they have the job and they're doing it for £5.
What sort of work do you do? Perhaps we can suggest places you can get work that will actually pay.
To get rid of such problems it is better to develop your own website, portray the sites you' ve designed, do SEO and marketing and take it to a level where people start recognizing you as a brand in days to come.
You wouldn't need much of an investment for that, now-a-days you get web hosting packages for as low as £2.50 GBP's so no big deal. The only crucial investment you'd need to do is time and efforts before the success begins to flow-in.
I know it isn't as simple as it looks, but the first few steps are important, once a momentum is achieved you only need to maintain and if possible increase the pace gradually.