This is a debate that could run and run. It's not really an either/or discussion, either. You should probably do both, but not in the same way.
If you know you should use social media better to market your business but can't find the time, there are professionals who can help. Check out our #HertsHour twitter list for PR and Marketing professionals and find the right one for your business.
Meanwhile, when you work out whether to focus your marketing on Twitter or Facebook, you need to find out where your customers will look for you, and what your competitors use. Don't reinvent the wheel!
Don't forget, although most social media is free, there can be benefits in paying to promote posts or advertise, if that's going to reach the right customers.
Whatever you do, remember to create great content. Don't waste time unless you have something to say. Boring retweets and posts that copy someone else's content probably won't sell your business to anyone.
It's all about creating relationships - the best businesses on social media are the ones that engage with their customers as people. For some shops, that means Pinterest and Instagram are best. For business to business, you might focus on LinkedIn. Choose what works for you!