Miss Watson is certainly presented as the more stern, nagging, disciplinarian of the two. She seems to be fit and determined to save little Huck's soul and mind by whatever terms necessary. Huck says that she started at him "with a spelling book. She worked me middling hard." It is at this point that the widow steps in and "made her ease up." So here, the widow makes her sister, Miss Watson, ease up on Huck a bit because she was working him too hard. Then the widow nags him about his posture, putting his feet on the furniture, and asks, "why don't you try to behave," followed directly by a lecture about hell. That's a pretty dire connection to bring up-telling Huck to behave and then immediately mentioning hell. Very nice, don't you think? She even informs Huck that there is not a chance that Tom Sawyer would ever make it to heaven either. Huck continues by saying that "she kept pecking at me and it got tiresome." After Huck gets his clothes dirty when he is out with Tom, Huck "got a good going-over" from her and then she "took me in the closet and prayed" for him. Lectures about muddy clothes, and being stuffed into a closet to be prayed over? That's pretty serious and weird. So, all of these descriptions (in chapters 1 and 3) seem to paint a picture of a super stern, nit-picky, strict, rule-enforcing woman who just won't let Huck alone. That's not a very flattering picture in the least.
On the other hand, we have the widow, who steps in for Huck's defense occasionally, and when Huck comes back after running away at the very beginning "cried over me and called me a poor lost lamb." And, where Miss Watson lectured him about his muddy clothes, the widow
"didn't scold, only cleaned off the grease and clay and looked so sorry that I thought I would behave awhile if I could."
And right there is a telling sentence. With the widow, Huck is actually motivated to "behave awhile", whereas with Miss Watson, he said, "I wished I was there," meaning hell, just to get away from her. What better evidence do we have of who is the more favorably presented character? :) I hope that helps a bit; good luck!