Just keep in mind that more POINTS does not necessarily equal more VALUE. I have NO idea what Jetblue or United points are worth but TPG (boy I hate using him as an example but I'll do it) lists the following:
Jet Blue = 1.3cpp // United = 1.4cpp // Southwest = 1.5cpp
Now one thing I am an expert on is Southwest points and I almost always get 1.6cpp out of my Southwest points. SO while yes you would get 10% extra points with United...it may still fall short of the value of Southwest. It seems that Jetblue would be a bad option based solely on these numbers.
Regarding Availability: Any hotel with 7 consecutive nights of standard award availability (search at Marriott.com) will allow you to redeem your 7 night certificate. I would assume the key here is booking early. Also if you aren't sure what category you're going to book in just get the Cat1-5 cert. for now and you can pay the difference (30k pts per category upgrade) later when you book your room......
Regarding the value of the points: Understand that the math here is basically this: you get to book 7 nights at a Marriott property and pay EXACTLY what you would have if you had just booked it on points PLUS transfer to an airline partner USUALLY at 1:1 (Southwest, Alaska, American, Delta, Frontier, Hawaiian, British, AirCanda, AeroMexico, Virgin, Iberia). The major exceptions are Jetblue which is 2:1 (pts cut in half) and United which is 1:1.1 (10% bonus points). A bunch of other foreign airlines are available at 10:7 ratio.
Using Southwest as an example...So you get your 7 night certificate at a Cat 5 property for 150k point - which again is exactly what you'd pay for a 7 night stay at ANY cat 5 property. Then you can transfer 50k, 70k, 100k, or 120k additional Marriott Points for Southwest RR points at a 1:1 ratio. Marriott points are typically worth somewhere between 0.8cpp and 1.3cpp in my experience and so now you are getting 1.6cpp out of them by transferring to Southwest. That's what makes this a good deal. AND You truly get the same value whether you transfer 50k or 120k or anything in between.
I've read numerous articles on these packages and very few seem to actually explain the math (the way I just did). IF you really think about it hard, they're simple to understand and it either makes sense for you or it doesn't. The big thing is if you can't do the 7 night stay in once spot or if that "cramps your style" then these packages may not be a good deal for you. I personally think they can make great sense on a
Disneyland trip if you plan to stay 7 nights in Anaheim. Happy to answer more questions if anybody has any...