Adam Dunn is worth a lot more in HTH than in Roto as are most low average/high power hitters. Since their average "resets" every week, you don't have to carry a .234 average all the way until September. Sure, some weeks suck...but even Manny was .148 and JD Drew .087 last week so Dunn's .234 or whatever gets lost in the week to week ups and downs and small sample sizes.
I disagree with this one. Even though a low-batting average hitter has his good weeks and bad weeks, over the 22-23 regular season weeks the team with batters that hit for higher averages will win batting average more times than a team of A Jones, Dunn types. I don't think HTH vs Roto affects these guys value in this way.
However, I do agree that the more consistent players have a little more HTH value as they help you more every week. Especially pitchers. Starters that have a decent overall ERA, but it's made up of great starts and blowups will hurt you more than they help you in HTH. Every SP has his blow ups, but when they happens it really hurts your ratios that week in HTH. A guy that blows up a few times versus one that does it quite a bit has more HTH value.
Another area of added value in HTH is the multi-positioned fantasy bench players that do one thing well. Whether that is high average (F Sanchez), stealing bases (R Freel), or hitting HR (T Wigginton) these guys can be great to plug into your lineup for a weekend series if you are close or behind in their good category and ok or really behind in other categories. For instance - you're dominating a guy in most offensive categories but you're behind in steals... plug in Freel for your power-hitting 2B or 3B. Or you've got 10 SB so far and it's friday - bench Chone or B Roberts for Wigginton and let him help you in HR.
The last value difference I've seen in roto vs HTH is a devaluing of pitchers in general due to matchups and the ability to punt categories. Say you've got a great starting pitching staff - an opponent could load up on MRs with good ratios, pitch the minimum innings and try to take ERA, WHIP with a shot at SVs from you, which would basically nullify - or at least lessen - your advantage. The same thing can happen if a team punts steals or saves... they can get power hitters and good starting pitchers without spending picks or auction $ on steals or saves and have an advantage on you in other categories. In roto this isn't a problem, as the guy punting categories usually hoses himself with the 1 point in that category and has not shot at beating you out for the championship.
Of course whether you have IP/Positions Starts maximums changes value also, but that's a different story.