Should I complete 2* collection?

You'll find many opinions, I think, but I honestly don't see any sense in using up 14 roster slots for 2*s.

You will see several threads here about 2* Farming, which is getting a full 13 covers for a character, getting them to level 94, making them a champion, and then reaping lots of fun benefits for each additional cover up to level 144.  The advantages are: quite powerful characters for their levels, lots of ISO, lots of tokens and 3-star covers, and then selling them and starting over.

My experience as a still-new player is that it's good to keep a handful of 2* champed characters, but don't plan to leave them maxed at level 144 and then use your 3*s exclusively in play.  If you're not using them as a farm, and you aren't playing with them in DDQ or in the required nodes in PVE, then they're wasting space on your roster.

Since, for example, 2* Captain Marvel will keep giving you occasional rewards of a 3* Captain Marvel cover, once she's at level 144, I will sell her (for the second time) and start again.  But I say that because I like 3* Captain Marvel and want more covers.

On the other hand, I like 2* Ares just fine, and am not interested in Ragnarok.  Same with Bullseye and Human Torch and their 3* versions.  So Ares/Bullseye/Torch all take up one roster slot for me.  Ares is using it now; if one of those other characters is required in the next PVE event, I'll sell Ares and add the other to my roster.  I have plenty of their covers in the 14-day queue to use.

Quebbster said:

Are you still relying on your two-stars in Daily play? If you are, it probably makes sense to focus on strengthening your Three-star roster for now.

However, once you have moved to the Three-star tier you will mostly use your two-stars to unlock essential nodes in PvE, and at that Point you may as well start farming the two-stars for their champion reward. Each cycle gives you several hundred HP, so even though it can be a steep upfront investment you'll earn that back and much more in short order.

This.

You're only really saving HP if you completely sell the 2* characters and that would cost you in PvE as well as DDQ. Otherwise by farming 2* you're earning 375 HP every cycle, making every 2* cover worth around 6 HP.

To echo what others have said until roster slots / HP become easier to throw away you're better off focusing on 3*s.  I recently tried rostering/champing all my 2*s on a side low level account.  It's painful to be constantly throwing away 3* covers because you can't use them and 2* farming doesn't build HP the way your 3* champs will.