Hi Judiray,
If your mattress is too firm and is still in good condition and doesn’t have any soft spots or impressions that have developed and you just need some extra softness and pressure relief then a topper can be a good idea. If the mattress does have soft spots or impressions and you are hoping to use a topper to “fix” it then your results may be disappointing and at best may only provide temporary or partial results and at worst can make some issues worse.
If you do decide to go in this direction then post #2 here and the topper guidelines it links to can help you use your sleeping experience as a reference point and guideline for choosing the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success and also includes a link to a list of some of the better sources for toppers I’m aware of as well.
Both Dunlop and Talalay come in a range of firmness levels so the choice between them would be a preference choice rather than a better/worse choice. There is more about the differences between them in post #7 here.
In general terms it’s usually best to choose a topper that is “just enough” in terms of thickness and softness/firmness to relieve any pressure issues you are experiencing and no more so you can maintain a good balance between comfort/pressure relief and support/alignment and there is less risk of the alignment issues that can come from having too much thickness/softness in the upper layers of a sleeping system.
It may also be worth considering a split firmness level with softer on your side and firmer on his side inside the same cover (if your mattress is a king size then this would be the same as two twin XL toppers).
Phoenix