adsl2+ has many advantages over adsl1. Greater efficency is one of them. Adsl2 is Adsl2+ but with only the Adsl1 range and without the extra frequencies it can still supply speeds upto 12mbit instead of 8mbit, I think the default sky is Adsl1 matt which would be a 8mbit limit.
If I use my line as a example I would gain from adsl2+ over adsl1, the frequency between 794mhz and 932mhz would be used since adsl2 allows single bit tones and is less wasteful, and as you can see my useable SNR looks to carry on past beyond the 1.1mhz mark, speedwise I reckon I would gain around a mbit, which to me is good but an isp would probably think not worth bothering with and since I have a 48db attenuation I expect I would be one of those people denied adsl2+.
Adsl2+ also increases reliability and reduces crosstalk so there is really no reason other then old hardware compatability to use Adsl1 instead.