Kontakt by Native Instruments would be my recommendation.
1. It is a software synth and it will work with any inexpensive midi controller/keyboard.
2. It is a sampling synth, which means it has sampled instruments in it's library and not just the patch based lower quality sounds you find with many synths that are lower priced.
3. It's in the $400 price range
4. It is professional quality and many pro's use it in their scoring work
5. It has the most after-market sound libraries out there and available at a very reasonable cost.
6. It works as stand alone or as a VST in a DAW
You want samples created by the synth mentioned? Kontakt as well as some other NI synths were used in these productions. These were before I got BB/RT
March To Tobith Rey-Al
http://soundclick.com/share.cfm?id=8968632End of the Line
http://soundclick.com/share.cfm?id=7202047 notes on EOTL drums are programmed in Jamstix using a Battery (NI drum)kit. and bass is Kontakt. I really enjoyed working up the guitar parts for this song.
Give Me A Chance
http://soundclick.com/share.cfm?id=10977017 Kontakt is used in this at 3:10 for the female choir in the background behind the little girl singing. The song is essentially done in BB/RB with Kontakt for the choir.
This is just a sampling of what you can do with Kontakt. It is a very versatile synth, adding the unique touches to a BB/RB production to laying down the drums and bass parts for a more conventional style rock/country song, or into a more full orchestral kind of thing with cellos and french horns and viola sections. It's limited only by your imagination and skill set.
I quite often will use a synth of one kind or another.... my 2 favorites are Kontakt and The Cakewalk Sound Center. If for example, BB/RB isn't giving me the kind of bass line I want in a song, the quickest path to the right one is to load the synth and play it myself.
The Caddy song
https://app.box.com/s/8zwr48mczf1isj64tnwu classic example of a bass line that is synth since I could not find a satisfactory RT that worked like I wanted.... it needed to channel Dusty Hill... as I recall, this is the CSC Ricko.
Hope this helps you a bit.