This is a really tough question, as we've been assessing the factors that make a Disney trip relaxing for years! Since my own childhood, I've stayed 1 night, 2 nights, 3 nights, 4 nights, 5 nights, 6 nights and 8 nights. We usually stay between 3 and 6. Like a previous poster, I'd rather go for fewer nights (3-5) but more frequently. We tend to take longer trips when we don't plan to return for another year or so.
But whether it's more relaxing really depends on a number of factors, and number of nights is just one of those!
Here are a few:
1) Having APs and visiting more frequently. That creates far less rush to do stuff and allows us to just visit our favorites.
2) When our boys (now ages 6, 13 and 14) understand that we will have down time at the resort. We were just at DL and I was on my own with our three boys one afternoon. They had the option of fitting in more at the park or taking a break at the resort - they actually chose the break!
3) When we stay between 5 and 8 nights. Eight is definitely relaxing, but we could easily relax with fewer nights if all these other pieces are in place. The only ones that are really hurried are the 1-2 night stays. Even a 3-night with realistic expectations (just MK and Epcot, for example) can be relaxing.
4) When we have ADRs for lunch and dinner each day. No worries about waiting in line, finding a table, etc. And we've selected places with healthy choices during our stay - good for everyone's overall health/mood during the trip.
5) When we're staying club level and don't have to worry about breakfast for five each morning. It also makes an afternoon break more enjoyable and then we visit the lounge for wine/beer/sodas and appetizers before a later dinner ADR.
6) The consolidation of Vinylmation trading. Now that trades are only available in limited locations, our boys are less obsessive about running into every store to check the trading situation, and we can all relax a little more.