Phd at same uni as MSc, possible offer but would like to change topic/group if possible I need some advice on how to diplomatically handle this situation:

I will submit my master's thesis in a few weeks, and I am pretty sure that I want to do a PhD afterwards. Research is what I want to do.

I have gotten a "maybe" from my current advisor (Prof. "A"), as in stay for a few months as an intern to extend/publish the work (minimally paid) and we'll talk again about a PhD (this is not specific to me, he did this with all his PhDs). However, there is a different Prof. "B" that does research on some other, more interesting topics, and if I could choose I would probably want to do my PhD in his research area. Although Prof. "B" knows me (positively) from a previous project, he'd probably also want me to intern at their group for a few months before making an offer. It is only during the course of writing my thesis that I realized that long-term the topics in Prof. "B"'s group actually interest me more.

The question now is:

How do I handle this situation, and maximize my chances of getting a PhD offer without upsetting anyone? I really don't know how to approach this situation, and I don't want to seem uninterested before I have something official. Do I talk to Prof. "B" in secret? What would I ask him? What do I do if Prof. "A" asks me to sign a contract for these next few months? Should I wait and apply for US unis anyways, regardless of my average GPA (see below)?

Some more info:


I am at a top-tier European university. PhD candidates are handled as employees and usually hired by the respective professors themselves. Candidates usually finish their MSc first, it's not possible to apply for a PhD position without having an MSc. Often, students working on their master's thesis will stay on with their advisor if the thesis is good enough. I'd say about 50% of PhDs get hired this way.
My grades are OK, but not stellar. Depending on the conversion, I'm at around 3.3/4.0, which is pretty much exactly the average of all graduating MSc students here. The university is known for not inflating grades, but this number still doesn't look too good.
I'm not too keen on applying to other universities: Most of the higher ranked universities are in the US, and I'd have to wait until December in order to apply for next year. Also, that would cost quite a bit of money, take more time, and most of the top unis have GPA minimums that are above mine (e.g. MIT has a 3.6 minimum).
Two partial answers:


It's not a problem for you to ask Prof. B about the chances of you doing a PhD for him. Even if it is in "secret". You are looking for employment, and you're not obligated to tell prospective employers about the other positions you are looking at. Any professor will assume that their MSc students apply for several (PhD or industry or both) positions, especially if the professor has given them just a "maybe" for continuing with a PhD.
If you get a positive answer from Prof B., I can't imagine there would be a problem saying to Prof. A: "I've been thinking, and I would prefer to do my PhD with Prof. B because I am more interested in researching Topic X, which his/her group is doing". If Prof. A's reaction to that is getting angry, you don't want him/her as your PhD supervisor or collaborator anyway.