It depends a bit on how much you use it and what you're planning to do with them. The Wacom Bamboo tablets are the best tablets for starting out. They're less expensive, and have lesser features than the other tablets, but are still very decent and fit for the average beginning artists. The Wacom Intuos are more expensive. They have more features and highter accuracy, but are imo only worth the money if you're drawing a lot with them.
No matter what you do, always go for Wacom. There are many other brands of tablets available, but I've never seen one match up against Wacom's quality.
I have a wacom intuos 4. It's my first tablet and I love it. <333 I have it for... 2 years? and It's still working fine. It's pressure sensitive, easy to use, and affordable (within the 100-200 dollar range). I use it with SAI and couldn't ask for anything better. XD I got it for Christmas too. We can be tablet buddies oh god //dies
I have a cheap 30 dollar tablet called Genius with a g-pen 560, if you are looking for cheap...(My older sister gave it to me, so I can't complain x3) However, if you want something good, I suggest wacom.
First of all, I think this question should be in the Hardware forum.
Anyways.... I personally prefer Wacom since they're the leading brand. I also recommend that company since I've seen that most of the other brands use batteries in their pens. Wacom doesn't.
If your mom is willing to spend less than $200, then you should look into the Bamboo line. They're cheaper and easy to get started.
There are 4 tablets to choose from: Bamboo Connect, Splash, Capture and Create.
Connect and Splash are almost the same but Splash has ArtRage Studio. But I guess it doesn't really matter since you plan on using Paint Tool Sai. Both cost $80 each. Capture has Photoshop Elements but again, it doesn't really matter much so I wouldn't go with this one. This one costs around $100. Create is the biggest and has the most programs. It costs around $200 so I'm not sure if your mom would be willing to buy this one.
So I'd recommend Connect or Splash. Splash is technically cheaper since it comes with 1 more program than Connect and the price is the same. But, since you plan on using Sai so go with whichever you want.
There are lots of cheaper brands that sell tablets for less (like Genius) but I find that Wacom is better in my opinion. Try to look around!
Waccom seems to be what most people have, the Waccom Bamboo is a solid starter tablet that's pretty cheap (80ish on waccom's site), and it can serve for quite a while. Personally I'd recommend that but I haven't had much experience with other tablet makers.