Very good post by Dark Shadows.
Having only seen bits of videos of them (and a few full games of Hull) I'm going mostly by what I've read and statistical dominance of their contemporaries. Here's my take:
Goalscoring: This is what Richard does best. But Hull is better. They are virtually even in terms of top-2, top-5, top-10, top-15, and top-20 seasons. Hull is 9-12-13-13-13, and Richard 8-12-13-14-14. But not all top-2s are created equal. Richard has five goal titles and Hull has seven. And Hull's goal titles were generally by wider margins against much stronger competition. That's before you add in some more credit for Hull's dominance in the WHA to whatever degree you prefer. Hull gets the edge.
Playmaking: Neither were ever the primary playmaker of their line, but they of course still had the opportunity to pile up assists. Richard was top-10 four times, never top-5. Hull did it five times, twice in the top-5. Hull gets the edge, though this does not mean much.
Hart consideration: Bobby Hull's best 8 seasons based on where he ranked in Hart voting are as follows: 1, 1, 2, 2, 3, 3, 3, 3. Richard's are 1, 2, 2, 3, 3, 3, 4, and, well, somewhere outside of the top-5. Clear edge to Hull here.
Toughness: Richard was known as a tough competitor and a good fighter who you didn't want to cross. Hull would fight, but was generally a lady byng-type player. No one ever called him soft. Neither was a big bodychecker. Richard's competitive drive hurt the team seemingly almost as often as it helped. I think Richard was tougher but I don't know that there was a major benefit to this.
Defense: Hull was not a defensive star, but I've seen him described as responsible, a very good penalty killer, a good backchecker, and his adjusted +/- from 1967-1972 in the NHL was very strong. Descriptions of Richard's defensive play have ranged from "it was bad" to "it wasn't what he did best" to "let's just not talk about that part of his game". No doubt, Hull wins this one.
Playoff production/team success: Richard was the top playoff performer of his time and a clutch scorer. He also had, by far, the most playoff PIMs of his time. He ended up winning 8 Stanley Cups. Hull only won one, but he was also the top playoff scorer of his generation too. A quick look at their best finishes in the playoff points race: Richard: 1, 1, 2, 2, 3. Hull: 1, 2, 3, 3, 4. In goals: Richard: 1, 1, 1, 1, 1, 2, 2. Hull: 1, 1, 1, 2, 3, 5, 8. Of course, being on a better team means you advance to the finals, and if you advance to the finals you have a better shot of making the playoff leaderboards. On a per-game basis it appears that Hull was every bit as offensively dominant as Richard individually, especially once competition is taken into consideration (Richard tore apart the war-torn NHL with 31 goals in his first 34 games) However, Richard did win the cups, and with Hull it is just a what-if. Richard has the reputation as a big-time clutch scorer, and though Hull is no choker, he didn't win like Richard did. Richard gets the edge not because he was personally much better, but because his team was, and he contributed greatly to that success.
Overall, I am sure that Hull should be considered the better player.
Ulf Nilsson was the same ***** center 
