I wanted both. All of this depends on budget. If I had to pick one, I would say focus on bass above 30hz as that is where the most bass is. If you budget is limited, I would focus on getting the biggest baddest sub I could with the expectations of adding a second one. You can always add more subs to increase output, but if your sub is not capable of producing strong output below 30hz, well, there is no easy cure for that.
$1K or so will buy you a really good sub that should be able to give you both, or at least, a good foundation in which to add a second (or more). Once you get into multiples, you will want to invest about $100 in a good microphone to measure your room and dial the subs in, and ideally, you would want to do this even with the single sub.
You have to take into account the room as well as that will also help dictate ported vs sealed, and budget. The bigger the room, the more you should plan on spending. Trust me, I had over 6000 cu sq ft to deal with and finally determined that DIY was the only way to go as I could not justify $3-4K or so in subs with three young kids and rising costs for just about everything. $1K gave me the output and extension of about 3 $1000-1500 priced ID subs.
Also, your mains will dictate how low you can effectively cross them over. I have found many manufacturers to be quite liberal with their specs...