The "interest" on SP is now very small, probably something like 1-2%/y (exact rate depends how much is powered up). That plus voting and curation rewards are the only differences between STEEM and SP.
Things were very different before the fork when SP earned "interest" of something like 150%/year. Holding STEEM rather than SP under those conditions was a huge disadvantage. It no longer is.
If you plan to just hold for a long term then it still certainly makes sense to hold SP given those advantages even though they are smaller (and I'm not rapidly powering down much of my SP, only a small portion of it) but the disadvantage is that it is locked up, can't be used for trading, and takes 1-13 weeks to access 7%-100% of it.