Sunday mornings are usually the quietest part of the week during Thorpe Park's peak period. You're correct, it will be busy whenever you go, but I've found that if you get there early (9:30ish) on a Sunday, you'll find that the park is relatively quieter until 11-12, mainly due to Thorpe's target market being teens and older families, who are more likely to lie in on Sundays. I've often found that on a Sunday, it's possible to whip around a couple of the busier attractions before the park gets too busy. Of course, being school holidays, it might not be quite so quiet, but it should be fairly good. In terms of the rest of the day, I don't see much difference between how busy the two dates will be. With that in mind, I'm leaning towards Sunday 6th, but there's probably not all that much in it.
I don't know how soon you have to decide, but as weather plays a big part, it might be worth holding off for 10 days or so and then checking the weather for the 6th. Of course, this requires you to plan your trip within a few days, but it gives you the chance to abandon the plan if a heatwave strikes.
Burniel | Great at theme park knowledge, not so great at designing signatures.