It's hard to split them as all rounders or batsmen imo. Watson always seemed a more reliable bowler, and he bowled an average of 6 overs per game, as opposed to Symond's 5. If Watson was in bowling form i'd have less reservations about him bowling at the death than Symonds.
As batsmen they played very different roles. Watson became a force at the top of the order, one of the best openers of all time imo, and Symonds played the floating role perfectly, spending considerable time in all batting slots and basically coming in ideally at around the 30-35 over mark to accelerate. Once again, there aren't many if any who were better at this than Roy.
Symonds does have the fielding string to his bow, he was a truly phenomenal fielder in the inner ring- most likely the best I have seen, and when we had Ponting, Clarke, Symonds and Hogg all in the inner ring it was something else.
Very difficult to split them, if I could only take one, I would usually take Symonds all things considered, but if the one you chose was required to bowl a full ten overs, I might pick Watson instead.