This old article
muddied the Thorens TD150/Rega waters originally...
If you weigh the tonearm then this site
should help you decide if its worth it....
I have had the good fortune to be able to experiment and try different arms on a TD150.
Between this one:Thorens TD150 Mk1 + Rega RB300 + Shure M97xE
, on Flickr
and this one:Thorens TD150 Mk1 + SME3009 MKII Experimental plinth (material and construction)
, on Flickr
The SME definitely sounded better to me.
There are a few caveats though. The cart I had on the Rega was not the best match for the arm, so my results were perhaps not the best reflection of the combinations possibilities...
What I can tell you though is that with both arms (Rega and SME) the cable dressing was massively important. I am not sure which Rega arm you have but if the cable is quite stiff then it is difficult to dress the cable so that it does not become a 'fourth' member of the suspension...
There are loads of happy people out there that find pleasure in the TD150/Rega 250/251/300/301 combo - so give it a go... Luckily the market for second hand Rega arms is quite buoyant so if it does not work for you the you shouldn't lose too much money (if any).