Hi,
As Mark says, there won't really be much (if any) difference in performance, telephone lines are not dependent on quality/distance the same way ADSL services are. The only difference you may notice is the call features offered by the two companies (such as BT's text service), but other than that you really shouldn't notice anything different on the way your line actually works. It really comes down to price, and the product you require. But there are lots of independent companies offering line rental and calls, and it may be worthwhile running a few searches to see what they can offer you in terms of line rental and call costs (although you could just take up line rental with one company, and pay for your calls with another) and products they can offer you on that line.