I have never used the Share Play feature but are you saying that if I do decide to share with a friend that I cant do anything on my console?

PS3 is still the best, allowing to share the game account between 4 people.

I only have a PS4, and all my games are bought with another person, as PS4 allows sharing only between two people (well, actually 3 can share a game with some messing around).

The way shareplay works is you can invite a friend to play with you online, or you can allow a person online to play your ps4. If you choose the latter, it is your ps4 doing all the work, so you can't do anything else while they're playing it.

I guess the DS download play was okay. Up to 8 players playing with just one gamecart. However, range is limited to local and games do not get saved.

The 3DS has the upgraded version of this but barely any games uses 3DS download play so I think that the DS sharing is better than the 3DS
Also, there is no transferring of digital games so sharing is not that great for the DS/3DS

I think we have a winner.  If you are a budget gamer playstation can actually offer you the cheapest experience be it what you said or a ps plus subscription.  I've never bought a map pack for call of duty on ps3 because I just signed in on one of my friends accounts and downloaded whatever he'd bought.  It's the best feature ever bar none! 

1.PS4
2.Steam
3.3DS

Steam's family share has been very useful, and I think it's the best balance for players AND publishers who pay to get the games made.

The PS3 account system was exploited by players and was one step away from piracy which isn't fair for the people who made the games. It's no wonder they have limited it down from 5 PS3 consoles to 2.