This question could bring you some pretty complex answers and formulas. Some FTers are very analytical.

I'm simple-minded, and, like you, I use Marriott and AA as my main programs.

Bottom line: a "good deal" is a redemption that pleases you. And that will largely depend on your goals and travel patterns.

With AA: I tend to use my AA miles for international travel in business class. My wife and I usually take one major international vacation per year, and we find business class seats so much more comfortable for these long flights. With AA, I'm trying to redeem at more than 2 cents per mile (the higher the redemption value, the better). The cost of the ticket divided by the number of miles needed to obtain that ticket gives you the cost per mile. For simplicity, I'll use the ticket cost that I see on aa.com or a search engine such as kayak.com. (Some will disagree with that, and say that tickets can be purchased in other, less expensive ways. And if you are able to do that, the cost per mile will differ.)

My job requires that I plan my vacations more than 3 months in advance. So I am usually setting up these major vacations well in advance. As an example, early this year we traveled to South America. A round trip J class ticket to my destination in S. America on AA "costs" 100,000 miles. It was priced at $5500 on aa.com at the time I made my reservation. That's 5.5 cents per mile. (Just remember that I might have been able to buy that ticket for less through other sources, or it might have gotten less expensive as my travel date got closer, so 5.5 cents per mile may be an overvaluation.)

Europe would be another example. Check the cost of a business class seat from the USA to Europe on kayak.com, then divide by the number of miles needed. The last time I searched, round trip business class seats to Europe from my base in Southern California were running between $3500-4000. At 100,000 AA miles for a J class ticket, that's 3.5 to 4 cents per mile.

So I consider a J class ticket to an international destination to be a "good deal." When flying domestically within the USA, I usually pay with cash and sit in coach.

Marriott: the game with Marriott is somewhat different. And it has recently gotten a bit more difficult. Again, if you look at my travel goals (one major international vacation each year), I think you will see that I prefer to redeem my Marriott reward points at international destinations. You generally get better value (in terms of points per night) with longer stays (5-7 nights). So I tend to redeem in places where I will have plenty to see and do for 5-7 days, which means destinations such as Paris, Rome, London, etc. Example: this year we stayed at the Marriott Plaza in Buenos Aires.

The best value when redeeming with Marriott is to use a Marriott travel package. That requires lots of hotel points. Travel packages bundle a hotel stay with additional airline miles that you can send to your favorite frequent flyer program. Unfortunately, Marriott and AA are ending their association as of June 30, 2010. Since I use AA as my main air carrier, this makes it harder, because I will no longer be able to redeem a Marriott travel package and get additional AA miles after that date.

Finally, back to simple things. I'm the one who makes all of our vacation plans. We're usually away for about 3 weeks on these trips, so there is a lot of planning. If my wife is happy with what I have set up, and if she enjoys the vacation, then I also think that I've gotten good value for my miles/points.

I have one more suggestion for you. Although I like Marriott, they do not have hotels in some locations I want to visit. You may want to consider adding a second hotel program. I use Starwood to fill in the gaps. Most of my Starwood points come from the Starwood AmEx credit card.