If I need to support 25 LWAPP AP's, then what specific controller series (4000 or 2000) should I go with?
You have a number of choices:
1. WLC 2100;
2. WLC 4400;
3. WLC 5508;
4. 3750G (standard PoE) with integrated WLC (25 and 50 AP); and
5. Cisco ISR with WLC network module
In my personal opinion, if you need to support up to 25 APs, you should be aiming for the 50 AP support (for growth). Which means that the 2100, 4402-50, the 3750G and the 5508 is a prime candidate. (Note: I'm not familiar with option #5 so I'm not going to get myself run-over by people who know more than I do. )
Next question: DO you need specific features like OfficeExtend and Wireless Plus? These are a few specific functions that are specialized by the 5508 only. If you don't need this then you have the 2100, 4402-50, and 3750G.
Next, as you mentioned earlier, the price of the 4402 doesn't persuade you. So this narrows your choice down to either the 2100 and the 3750G. I'm no big fan of the 2100 so I'll leave the decision up to you.
With only one WLC to contend with, you don't need WSC unless you want reporting and schedules. One of the benefits of WSC in a SME is the function of shutting off the radios outside business hours.
The 2100 is a very cut-down version of the 4400 (heck, except for the 5508 everything else is the 4400 but hardware/software disabled or limited) and it's very slow. It's not designed for APs that are intended to run 802.11n because of the 10/100BaseTx interface. There are some functions, like LAG, that are not supported.
The only time I would have the 2100 is when I'm have a test environment or a demo. Otherwise, I would always recommend a WLC 4400 as the most basic stuff and move up.
One scenario when 2100-series can be used, and I have installed some, is in warehouses. Here you might have a very large area to cover, but very few clients, like forklifts och handscanners, sending very little traffic. You still would use the n-standard to overcome multipath and reflection. In this scenario 2100-series work fine and will save some money.