The lesson I've learned from the various reserved instance reductions is to never buy 3-year terms.
1) the 3-year reservations have a lower per-hour pricing than 1-year
2) the 1-year instance you would purchase a year later will have a lower per-hour price than the 1-year today.
3) you can resell the 3-year instance on amazon's marketplace after you're done with it, so it has residual value.
So it all depends on how much prices drop, what resale values on the "old instances" are, etc.
All in all, the differences probably aren't huge unless you're at massive scale (eg Netflix, Heroku, etc). But at that point you probably negotiate with Amazon directly anyway instead of working off a pricing table.
$price = $reservationCost+$hourlyRate*24*365*$numberOfYears
#2 is sort of what we're solving for... how much of a yearly price reduction would be necessary to make 3 consecutive 1-year reservations cheaper than a single 3-year reservation (or how much of a 3-year reservation you could effectively abandon and still come out ahead). By my math above, I came up with the magic number being about 40% discount per year making 3x 1-year reservations cheaper than a single 3-year reservation.
$765 + $765*x + $765*x^2 = $1515
x = 0.60923
By purchasing 3-year RI's prior to the utilization categories, I paid more (350 vs 300) for the reservation than even the costliest new reservation type (heavy) but my hourly rate is higher ($.06 vs $.059) than even the costliest new hourly type (light).
At this point I can't run the numbers, though, b/c I'm not sure how much a 1-year term would have cost me back then. I only know how much I did pay for my 3 year terms.
Maybe my gripe is simply that I feel my pre-category RIs should have been converted to "high" RIs with better hourly rates =)
The lesson is more that you should pay attention to when the last reduction occurred for a given service, and the actual discount you're receiving for reservation. Another good consideration is how much of your time it takes to determine cost analysis against the fact that you're probably investigating cost savings in the tune of +/- 5-15% (EDIT: leaving this here, but this number doesn't make a lot of sense) of your total aws charges anyway.
Of course, YMMV.