It's never easy to make a decision like that one, but my advice is if your unsure don't rush into it. Does your bf have anywhere else he can stay besides with you?
Another issue is that your in touch with your ex which is only going to make things even more difficult for you! I totally understand where your coming from though, your only weighing your options, and what women doesnt want to feel secure financially. But your dealing with someones feelings and you dont want to lead your bf or your ex on.
This is a situation in which you have to weigh your pros and cons, because even though you will feel more stable with your ex he is your ex for a reason, and you been with your bf for 3.5 years thats a long time to be with someone then ditch them because there in a rut, you should probably stick around to see if he will do better but dont let him move with you because if you decide hes not the one it could be hard to get him out