I think increasing their equity, even if only a small amount, is a gesture that will garner goodwill with your early investors. Cultivating an attitude of "we are in this together" cannot be a bad move for any startup.
Based on what? The 1st investor should know better. They should say, "we believe in you, and we want to offer the additional funding." This is not a game. If OP has something that investors are interested in then they should be able to raise cash for the good of all shareholders. The OP should go to the original investor and offer them the first right of refusal. This is a business. The angel should say, "cool here is the 150K" if they believe in the company. This is either a new angel that has have never done this before or an angle that is trying to take advantage of a new founder. The right thing for the OP to do is to tell the investors they need addtional funding and ask if they want to particpate.
For what it is worth I have invested in angel rounds, arranged for a startup to receive angel rounds, done a few startups and have held board seats. Also to the OP - warrants with a note is something to look at.
Nobody is implying dealing with investors is a game. Communication is key. I guess I assumed the OP did communicate with the original investors. My mistake with that assumption.
Let me clarify, I am not suggesting that you simply give someone stock because they grumbled or feels slighted because 2nd round angels received more value for their investment. Maybe the 2nd round investors were simply better negotiators. There are a number of reasons that giving more stock was necessary. I am suggesting that maybe the first angels have abilities etc that could have helped to hold costs down etc. requiring the necessity to raise a smaller amount which would ultimately minimize their stock further diluting. This in effect results in greater stock value than if you would just take the total amount of money from additional angels.
Angels I have worked with have always been great and would not expect a "gift" of additional stock. That being said, if an angel had skills, products, or services that I otherwise would have paid more for, I worked with them to supply what was needed for far less $ and future stock options.
Experienced Angels realize that raising more money is often necessary and if additional money is not raised their risk on their investment increases exponentially. Further, most would have first right to increase their investment so they can invest and prevent their dilution as well.
Communication is the key and I was fortunate to work with good angels. I had their back and they had mine, hence we were in it together. They never expected to be given anything but because we communicated their skills and networks became readily accessible.
For what it is worth I have invested in angel rounds, arranged for a startup to receive angel rounds, done a few startups and have held board seats. Also to the OP - warrants with a note is something to look at.