Hi there -- Thanks for your question. At 300 lbs and up, I do think the options start to get rather narrow -- particularly if you're looking for a mattress that will hold up for 5-10 years. IMHO, you'll want to look for product lines that are specifically designed to be supportive (and durable) for heavier sleepers.

One that I think is worth checking out is the WinkBed Plus, which you can see here: https://www.goodbed.com/mattress-model/winkbed-plus/ On this page, you can see our review of the regular WinkBed. The WinkBed Plus is a heavy-duty version of this mattress that is specifically designed for people 300 lbs and up.

Another good option that is sometimes available on Amazon would be the Medicoil HD (Heavy Duty) line here: http://amzn.to/2tlYZtj The coils in this mattress are made with 12.5 gauge wire, which is extremely thick as mattress coils go. Likewise, the foams are high-density, durable foams that are specifically made for people in your weight range. You can even get mattresses in this line that are flippable, which increases their longevity (and reduces body impressions) even more.

Apart from those specific options, here are some more general things you'll want to consider:

• If it's an all-foam mattress, the base layer needs to be at least 2.2 lbs density.
• If it's an innerspring mattress, look for lower-gauge firmer coils. Short of being able to find this, connected coils would be better for you than pocketed coils.
• Latex is also a good option with proven durability, particularly as a comfort material but also as a support material.
• A mattress that's at least 12" thick will generally be better for you to keep from bottoming out, though thicker than that is not necessarily better.
• Keep in mind that in most cases a given mattress will feel a bit softer to you than it would to someone who is significantly lighter. The exception to this is a mattress that has soft layers on the top with a very firm layer underneath -- in this case, the mattress could actually feel firmer to someone heavier (e.g., if they sink through the soft stuff and are resting right on the firm layer).

I hope that's helpful for you.