How long are your classes? Assuming they are 100 minutes, that is going to stretch the battery life of almost anything, in which case a removable battery (that you can throw in your backpack) is going to be key. In your position I would probably go for the Dell. There is no way to guarantee a single battery will last that long, and even if it does once it degrades (which it will relatively quickly since you are doing deep discharges every day) battery life will be lower.
I honestly think that the inking performance in Office 2013 is good enough to use on a daily basis, and there are some definite things you can do to boost performance. Configuration is very important, and it is more likely lags are being caused by configuration than the wacom digitizer itself.
1) Keeping a clean installation image is very important especially with an Atom-based PC.
2) Disabling background indexing can help (though I don't because I like to be able to search through notes in real-time)
3) Disabling touch (not pen) input can help a ton. (Most of the lag I experience is the tablet trying to integrate my finger touch movements and the pen input. Onenote does a very good job of differentiating the two and making sure your strokes appear where they are supposed to, but apparently the processing power necessary to make these adjustments is pretty high and bursty, leading to lag).
If you can, head to a nearby Microsoft Store or similar (maybe Staples?) and play with a TabletPC. At this point I still consider them niche products but I believe we are on the verge of them being mainstream devices that will be commonly found in classrooms.
As an aside, (since many people do not know of this feature), if you record audio while you are inking, you can replay the audio at any given stroke point, which is incredibly awesome. That also takes up resources though. I still do it and it is easily workable. #3 is still the biggest lag generator for me.
Edit: More on the battery: Lithium-Ion batteries tend to like shallow-discharges best. I used to be in a similar position to you back in the day and my batteries did not last, because I was charging them up and then completely discharging them. This is why removable batteries for this type of application are a good thing, especially if you know you are going to be bumping up against the max runtime limit.
Edit2: More info on the above here: http://batteryuniversity.com/learn/a...ased_batteries