james, I know some of the people at Hoare and am therefore interested in your predicament.. you said that Hoare Capital are targeting niche markets which are not available for screen trades, and considering they are recruiting for a sales trader role this doesn't seem to match very well with your aspirations to become a 'screen trader'.. It seems to me that the role you are looking at is much more about sales and broking rather than executing trades.
With regards to the uncertainty about the firm I personally think that joining a young firm has massive benefits and opportunites although the risk is substantial. However this shouldn't worry you so much considering you, like me, are at a very early stage of your career.
If I were you I would think really carefully about where I want to be in 5 years time and backtrack from there. I completely agree with GammaJammer in that this will be a very specialised role and will most definitely limit your options in the future (although it will open many doors within its sector!!)
ps, have you discussed your qualms with Hoare? they might be able to make the distinction more clear-cut