The Aspire is a key system, w/ key system limitations. Which these days usually is just related to growth. Key systems have pretty much the sames features as a PBX & sometimes more.
The NEC 2000 IPS is a PBX & you pay for that.
Intertel(a key system) has always been pricey.
3Com has its moments where it is useful, but you are buying the name. I, of couse love the NEC 2000 IPS. I use the IP funtions of the NEC & get great reviews & results. These are, of course, just opinions.