You can compare the rates and volumes of the various exchanges that trade XRP at http://coinmarketcap.com/currencies/ripple/#markets

Note that trade volumes within Ripple's internal distributed exchange dwarf that on the external exchanges while giving better prices. To see how internal trade volume is segregated, visit www.ripplecharts.com for an overview / per currency breakdown and www.ripplecharts.com/#/markets to examine individual order books and trade histories.

Note: ripple charts might be a little slow right now. Western Union just announced that they are engaged in a ripple pilot, and that has caused a surge of traffic to the charting site.

To find out more about individual gateways, visit www.coinist.co

The gateways with the highest XRP/BTC volumes will be Bitstamp and SnapSwap (SnapSwap is listed as btc2ripple in many places). SnapSwap-btc2ripple is what you receive if you use the inbound Bitcoin bridge from within the www.rippletrade.com client (under Fund->BTC menus/tabs).

One nice thing about bringing BTC into Ripple through the inbound bridge in RippleTrade is that it will auto convert a few cents worth of BTC into the XRP needed for your account reserve if your account has not already been activated.