Which banner should i roll for summer

Roll for Squirtle but if you don't get her Kuro is going to be coming up in the next event as a welfare SR Archer consolation prize

Here are some numbers which might help you decide:
- Chance of getting Kiyo: (1.5/3)% = 0.5%
- Chance of getting Tamamo: 0.7%
- Chance of getting Martha: (1.5/2)% = 0.75%
- Chance of getting Archuria: 0.7%

Source: https://www.reddit.com/r/grandorder/comments/89nzhb/after_nearly_3_years_dw_finally_publicise_the/

Also consider that we get Chloe in a few weeks as a welfare servant and she does basically the same thing as Archuria.

If you want to roll purely based on how much you like each servant, the Archuria banner will be better. If you're taking gameplay into consideration and are willing to wait a few weeks for Chloe, the Tamamo banner is better, since good ST lancers are much harder to get than good Archers.

Since you are saying that you lack a good archer and lancer I guess you are thinking about rolling gameplay-wise, so my advice is to roll for the part I event, there you have a chance of getting Tamamo-lancer or Kiyohime-Lancer, that way you may solve the lancer problem, and for the archer problem you can wait between 2-3 weeks and do the event of Prysma Illya wich has an archer servant as reward for completing the event (Chloe/Kuro is top tier ST archer), so you may have both servant departments covered.

Also in part I you may get all the servants you need cuz there are 3 classes (Lancer, Archer and Rider), so higher chances to get what you want/need.