Reddit reviews Genuine Sega Original Saturn Controller Model # Mk-80116

http://www.amazon.com/Retro-SEGA-Saturn-Classic-Controller-Pc/dp/B002LHS70K/ref=sr_1_1?ie=UTF8&qid=1348868892&sr=8-1&keywords=sega+saturn+usb

These are gtron branded, I haven't played on them. This is the one I had and abused for many years:

http://www.amazon.com/Genuine-Sega-Original-Controller-Mk-80116/dp/B000P9GQ7W/ref=sr_1_6?ie=UTF8&qid=1348868892&sr=8-6&keywords=sega+saturn+usb

If Sega put out an official USB one, I'd consider buying it. If you really want a pad over a joystick, the PDP one received good reviews. I used the MadCatz one and it wasn't all that great after a few hours of abuse -- I'd stick with a stock PS3 controller over it. Here is a review on that unit:
http://kotaku.com/5767903/the-marvel-edition-versus-fighting-pad-is-hideously-wonderful

Sadly, the reviews of the Gtron look poor and many people recommend buying an adapter (which I wouldn't do). Not many solid options for current gen consoles + gamepad + fighting games.

I'd probably just get a PS2->PS3/4 converter from Brook and a PS2 controller (not sure if PS1 works). The only thing I have against PS3/4 is that the trigger buttons aren't really ideal for fighting games and PS1/PS2 controllers are cheap and good quality (Luffy won Evo with a PS1 controller I believe). It's the cheapest way to get a high quality pad that'll last. I'm not sure if you have to update the firmware on the converter whenever there is a big PS4 update. For Brook's fightboards, if you don't update the firmware, you get stuck with an 8-minute timeout. Brook has been very good at updating their products' firmware within a day of big PS4 updates so far though.

I myself got frustrated with fightpad options, so I built a converter for a Sega Saturn pad (what the Madcatz pads are based off of, but better build quality). I had to buy an FGWidget Converter board but they don't seem to be available for sale anymore. Then I wired the FGWidget to a Brook PS3/PS4 board. It was a lot of work and soldering, but it's flawless, and I can switch out Sega Saturn pads as needed, though I haven't had to in years.

(BTW, I play on gamepad because I have wrist issues so I can't use fightsticks for very long)