First, for example’s sake, let’s pretend you just added 100 subscribers to your subscriber_all data extension, and you know at least some of them qualify for the filter/filtered data extension you are sending to.
If you send an email to a filter, the filter is run at the time of send. Therefore, those subscribers you just added who qualify for your filter will be included on the send. A downside of targeting a filter (in my opinion) is that you won’t know how many subscribers will be included on your send.
If you send to a filtered data extension, the data extension would need to be refreshed before sending the email in order for those new subscribers to be included. However, by refreshing the filtered data extension, you can see how many subscribers the email will send to.
If it’s a one-time send, you are probably safe setting it up either way (so long as you refresh the data if you send to the filtered DE!). For reoccurring, automated sends, you are more likely to want to send to the filter itself.
I hope this helps!