Two offers feel like a decision you should outsource to someone wiser. A mentor who likes you will still not live your summer. If you ask them to pick, you will get a hedged paragraph and you will still be the person who has to answer the recruiters.
Ask for an opinion: which risks they see. Keep the choice.
Opinion, not a verdict on your life
A decision transfers ownership. An opinion transfers judgment and leaves you holding the outcome. Two internships are the second kind. A busy expert can tell you the failure mode they have seen. They cannot honestly own "take B."
Write the two offers as a cold reader needs them: team, location, dates, pay if you are willing to share it, the work you would actually do, the one thing you are worried about in each. If a claim is marketing language you have not verified, label it.
Do not send the spreadsheet of twenty weighted criteria your assistant built. That is your homework. Sending it asks them to grade a model.
Ask for risks, not a rank
"What risk am I underweighting on offer A, and on offer B?" is one question with two named objects. "Which should I take?" is a different question and the wrong one.
A sufficient reply is a lean plus one reason, or "I have no useful view." Say that the second answer is allowed. Mentors delay when declining feels like abandoning you.
Give a date that matches a recruiter deadline, and say what you will do if they stay quiet: you will decide on Friday and you will not wait. That is courtesy, not pressure.
Your assistant has opinions. If the draft already sounds like you have chosen B, your mentor will argue with a choice you have not made. Cut the verdict. Keep the facts.
You still send it
Draft. Read. Confirm. A note that uses a mentor's time under your name is not auto-mailed.
They do not need an account. Email and a link are enough. If they already have RelayLink, you need an accepted contact.
No attachments. Offer letters stay in your files. The briefing is the comparison and the ask.
Nicknames stay private unless you share them. Write the name they use at work.
If they block you, the block is silent. Decide on the date you named.
When the two offers fit in a briefing they can answer on a train, connect your assistant and ask for the risks — then you pick.