I'm a newbie to the forum - thanks in advance for any thoughts that might be shared...

Need to upgrade my tripod/head gear - B&H recommended the Manfrotto 504HD Head w/546B 2-Stage Aluminum Tripod System (now on sale under $600)

I've been looking at the Sirui VH-20 Video Tripod Head BSRVH20 and would pair it with either their Sirui VT2203 Carbon Fiber Professional Video Tripod or the aluminum model.

SIRUI VH-20 SERIES PROFESSIONAL VIDEO HEADS (http://www.sirui-photo.us/Sirui/Sirui-VH20-VideoHead.html)
SIRUI VT SERIES PROFESSIONAL VIDEO TRIPODS (http://www.sirui-photo.us/Sirui/Sirui-VT-Tripod-Series.html)

Any one have any thoughts about which way to go? Anyone use the Sirui gear? Any other recommendations?

Camera is a NXCAM NX5U, with varizoom controller, shooting equine events, clinics, shows. So need to be able to have a butter smooth pan 200-300 ft out.... shoot sequences can be very long and they are live. Currently using Manfrotto 501head with Manfrotto 3021 legs, the nx5u footage looks jumpier than when using the consumer HDR-SR11.

Carol,
Welcome to the best forum around.
To answer your questions, I have never heard of or seen the Surui brand so I can't really compare it to anything but I have owned many Manfrotto legs and head over the years. the 2 set of legs I currently keep in my grab box are the 3246 legs which are a center column aluminum leg with a 501HDV. Normally this is used only for a lock down but I like the legs because if I have to I can get them up to 8 feet high with the column. Good for shooting over things like a crowd of basketball players.
My main set of legs is the older 525 which I believe the 546 replaced. I have been using a 504 head since about the day it came out and frankly I haven't had one bit of trouble with it. I've used all kinds of camera rigs on it from 5 pounds to 25 pounds and while I don't recommend a 25 pound rig on that head it can be done. Using the NX5 loaded out should be no problem for the head and legs BUT keep in mind if you're out to the long end of the glass ANY wobble of ANY kind will show up but that's pretty much true of any head/legs unless you're talking about say a Cartoni that you need 2 people and a small pack mule to move.
It seems to me that for what your needs and appearently your budget I think you'd be pretty happy with the Manfrotto rig.

The Manfrotto 504/546 kit is certainly an instant classic -- I'll probably go with the 504 Manfrotto head for sure... the Sirui/Manfrotto stick design seem to be quite similar - and the carbon fiber price point on the Sirui gets my interest... is it worth it to invest in carbon fiber made sticks?

Thank you Don for your welcome and sharing your experience with me.

I've never owned a set of CF legs but I have used them and while the weight is much lighter than aluminum I found there was more twist in the legs than even the 525 legs with a very heavy load on them. Maybe it was that particular set of legs or that particular camera rig (it did go out at about 28 pounds IIRC).
In any case I like my 525/504 combo.