If you are looking to replace fixed internet with mobile, and mainly use it in the house, you are better off changing your fixed provider. Unless you are very close to a high speed 3G cell site then mobile will generally be slower indoors than a fixed connection.
If you are looking at mobile because you want to use it out and about, then it depends where you are – different companies have different levels of coverage, and different strategies around their networks. Vodafone has a good level of coverage across the country, but focuses more on big urban areas for the fastest speeds (where they tend to roll out the fastest kit much more than the other providers). Orange gives wider coverage (they claim) but it's not as fast.
So, as with many things – it depends. I've got a Vodafone one, and I tend to use it in places that have the faster service (HSDPA 7.2 and above, to be slightly technical) – and it's often quicker than my fixed service at home. If I was using it out in the countryside it's unlikely to be that fast (equally other networks wouldn't be either).