Hello Paul
When I bought my ST1000+ (2009) the ST2000+ was over £100 more expensive. With a difference of only £35 I would opt for the ST2000+.
I believe that the two units are electronically identical, but differ mechanically. The ST1000+ uses a simple screwed rod and nut to generate the thrust, whereas the ST2000+ uses a recirculating ball mechanism, which is much more efficient.
The ST2000+ will use less power for a given thrust. You can download the manuals for both models from the Raymarine website to make a proper comparison.
If you have read my previous posts on this subject, you will know that my unit suffered magnetic interference from the outboard motor (Honda 5) , as did my Simrad tiller pilot on my last boat fitted with a Yamaha 4.
As Charles has mentioned I got round this problem by fitting a remote fluxgate compass, this of course increases the cost further.
Frank Dierkes (Popeye) had the same problem with his Simrad and a Suzuki outboard.
If you look at my albums you will see that mine is simply mounted, with the supplied bush epoxied into the top of the cockpit coaming. With this arrangement I had to use a thrust rod extension of (I think) 150mm.
Having said all that, I would not be without mine. It gets a lot of use as I am usually sailing solo. It enables me to hold the boat head into the wind (with the motor) to carry out mainsail handling in safety. Of course it also allows me pop down below to put the kettle on etc when under sail.