Hi Psyotic,
The choice between memory foam and latex is really one of preference and both can be used in various layering combinations to provide good pressure relief and support/alignment. You can read about some of the pros and cons of memory foam here and some of the pros and cons of latex here. Your third foam option would be to use a high quality polyfoam in your comfort layers which can be a lower cost alternative but if the quality/density is too low then it may not last you the 3-5 years you are looking for and keep the properties that provide you with the pressure relief and alignment you need. One of the main differences is that latex and polyfoam are fast response materials and memory foam is a slow response material and different people may prefer one over the other.
I think highly of Alan at foamorder.com and he uses good materials. Some of his products in the clearance are very good value. If you are local in the San Francisco area … this would be the best of both worlds because you could actually test the mattress (which would lower the risk compared to an online purchase) although I would want to know the density of the memory foam and polyfoam. No matter what the value though … the most important thing is that it suits your needs and preferences and either testing the mattress (if you are local) or a conversation with the manufacturer along with the results of any personal testing on similar mattresses to make sure that the odds of it being suitable for you would be important IMO. This is especially true when it is a final sale. No matter how great a mattress is in terms of quality or value … if it doesn’t provide for your needs and preferences then it would have little value for you.
A search on foamorder (you can just click this) will bring up more comments about them.
Kopavi has mentioned a couple of very good value options (thanks Kopavi :)) which have good value in latex (Ultimate Dreams is new and Mattresses 24/7 uses seconds and comfort return materials from a good source).
Post #12 here also includes some of the better memory foam sources and a few options that may also be inside your budget.
If you let me know the city you live or your zip … there may also be local manufacturers close to you which also offer good quality and value options in either memory foam, latex, or higher quality traditional innerspring or polyfoam mattresses.
Phoenix