Storj and SIA are different, furthermore the teams behind them have different visions for the projects, in the case of SIA from what i can see they are more focused on being as decentralized as possible, even if that goes at the expense of performance. Storj is more focused on bringing high performance object storage which traditional companies can use more easily, which goes at the expense of decentralization, so Storj is much more centralized as SIA. That 6PB (now close to 8PB) doesn't have any real adoption value, it was meant to test if Storj can scale, which it can.
The thing going for SIA right now in terms of development is that their team has been rock solid and growing over the last few years so they are crunching the numbers very fast. Compared to Storj which has had some leadership/management crisis a few months ago which has set the team back a few months, though thanks to the 40 million dollar token sale there has been allot (and there will be allot more) on-boarding of developers, which will continue over the next year(s).
From a personal point of view i like both, i farm both and upload to both. The thing i dislike most about Storj right now is the bridge that isn't stable and causes me issues, since there is only 1x bridge at the moment, if it goes down, so does the entire network. The thing i don't like about SIA is although they are more decentralized, any company interested in using SIA will make it more centralized, as you can't force consumers or businesses to first sync a entire blockchain (which takes ages) before they can upload data, its just a pain. If adoption were to skyrocket, you could (and will) get into a situation where anyone interested in running SIA to store some files will first have to download a +100GB blockchain before they can store some files, which is nuts. Though it appears in a few years there will be a solution for this.
These are just a few advantages and disadvantages of both, so yeah. Personally i think its smartest to use both as you can leverage between each project's strong points.