Well as nobody else has replied.
I bought my first bivvy last year so have nothing to compare it with.
Its a JRC Contact single person. I only wanted a small one for occasional nights in summer and it did all I wanted of it this year.
I beleive they can be had at around £100 but got mine off ebay as new for £50.
Another vote for the JRC Contact. I spent last weekend in it and it kept out the rain and wind on Friday night a treat. Some slight condensation was experienced, due to it being a single skin (although you can now get an overwrap for it), but nothing too serious.
Got a chub s max plus with overwrap which I have given intense useage this season and it has not let me down. I only got the overwrap a couple of months ago, but without it it is well in your price range.
It is versatile as the front can be totally removed to create a brolly effect and it has protected me from some truly horrible weather since last march when I got it right up till a couple of weeks ago when it iced solid.