Loyalty v career progression

This is primarily aimed at the senior members of this forum. After a year of recruiting I managed to get an off cycle internship in an up and coming no name IB boutique ( 20 employees with strong deal flow, starting to win massive M&A mandates in our area of expertise). I impressed the MDs and was recently offered a full-time position which I accepted.

Now a few months down the track, I have been offered an ECM position at one of GS/MS/JPM by a contact I got to know last year. I am well paid, being mentored extensively and given massive responsibilities at my current employer and I have great relationships with my MDs. The issue is, in 2/3 years I'm not sure if I will be able to move to BB IB with an unknown brand name. I think I will be able to move within the BB to M&A from ECM pretty easily in a year or so. I'm really struggling to decide whether to jump to GS/MS/JPM now or stay with my current firm.

I have got really close to my MDs and want to stay loyal to them. They gave me my start, took me under their wing and have put heaps of time into training me up. Loyalty is really important to me personally and I don't want to let my current firm down, but at the same time my dream for a long time has been to work at a BB (I want to stay in banking long term). Can any senior members give me any advice on what they would think if one of their subordinates who they helped out massively jumped ship after only a few months.