Polo has two currencies that are pegged to the dollar.
Steem Dollar is one of them i would use because they pay interest of like 9% right now.
You have to put them into your steem account to collect the interest, but thats super easy.
A few comments on this:
1. As you noted, you only receive the interest if your hold the coins in your own wallet (using the steemit.com web wallet counts). If you keep the coins on polo, polo gets the interest.
2. The interest rate is variable and has been trending down. It is currently 6%. You can see the current rate in the sidebar on steemd.com:
sbd_interest_rate 6003. Steem Dollar (SBD) is backed by the value of the Steem blockchain. If STEEM drops enough in value SBD holders will also take a loss. According to blockchain rules this starts to happen when the market cap ratio of SBD to STEEM increases to 10% (currently about 3%) but the market also plays a role. SBD can and has fluctuate in value, though recently it has usually been within a percent or two of USD.
The other option is USDT which is backed by USD in a bank account held by the company that issues USDT (Tether). It is exposed to the risk that the company can not or does not perform in redeeming the USDT.