1 tile rarely makes any difference. Usually the tiles that matter are 3/6/9/12/15 as they most often come in groups of 3. Now 7 tiles vs. 8 does matter more often as a 4 match happens way more frequently than a 5 match. With that said, overmatches (a cascade triggered) benefits the slower hero than the faster one usually.
So really you are comparing:
| # of tiles | Hero charged |
|---|---|
| 3 | None |
| 6 | Very fast + 9% mana, Fast + 34% |
| 9 | Very fast, Fast, Average + 12%, Slow + 34% |
| 12 | Slow, Very slow + 13% |
| 15 | Very slow |
* Not including Ninjas
Percentages taken from this great post: Troop Comparison & Summary