[item]Thrash Blade[/item] provides more dps against casters. Also, the weapon speed is largely irrelevant since the weapon swings you get deal more damage individually. Your chance on hit procs also normalize to weapon speed regardless of the extra swing possibility. You'll also find thrash blade more powerful against casters, when fighting whom you often don't have the reckoning buff. And even with the Reckoning buff, since it triggers off of taking rather than landing a hit, you can still often get 3 extra swings in. And while the extra swings can't trigger thrash blade, your white swing can use a reckoning swing and proc thrash blade at the same time, making a triple-hit.
The reason thrash blade is often overlooked is because many players overlook the possibility of adding a new proc or ability when considering other items with juicy stats. The fact is that the rest of your gear gives good stats, but where else will you get something like the thrash proc? A little strength and stamina will rarely make the difference, but an extra weapon swing easily can.
If you plan to go offensive with your prot pally, thrash blade is what I found to be the best. If you're just flagrunning, [item]Viking Warhammer[/item] will be best as was already mentioned. Offensive prot pallys are fun but I understand the reasons to focus on the FC role instead.