There is no one answer to that, it just depends on how much money you want to spend and how fancy you want to get. On my truck I use a Comtelco with a mag mount, I can't really speak to range because I use the GPS to lock out things when I get that far from them. I will say the Comtelco is a good deal for the price, you can get them from Scannermaster, but you'll need an NMO mount no matter how you decide to mount it (mag mount, trunk mount, etc.). At home I have a Larsen tri band on a Tram ground plane kit about 10 feet off the ground, and even with a roughly 100 foot run of coax (75 of which is LMR400) I can reliably hear out to about 45-50 miles. Prior to the Larsen I had another Comtelco on that mount and I can tell you on VHF and 700 & up the performance is almost identical, but on UHF the Larsen kicks the Comtelco's butt up the street and back again. Unless you're trying to hear over into like McDonough County or something further away you likely wouldn't notice this. Again, both these antennas would require an NMO mount of some kind. If you go with the SDR, you will need to get adapters for the coax, the SDR will either have an SMA or MCX connector. My interior cable runs use regular TV coax with F connectors, because I'm not using long enough runs to notice the loss it has at higher frequencies and it's much cheaper, easier to find, and easier to work with than the LMR400 I use outside. Either the Larsen or Comtelco combined with a Tram ground plane make good base antennas and are probably small enough you could keep them inside and put them on top of a desk or something if you wanted to.
I would say if you ONLY want to hear SC21 at one time, go with the SDR. If you want to be able to scan multiple sites at once or scan both SC21 and conventional VHF/UHF, go with a 996 or 325. The SDR has the inexpensive factor, but what you save in price you give up in flexibility. That's what I would say anyway.
And as always with antennas, your mileage may vary. I live at the top of a hill, so my antennas are going to work better than someone who lives at the end of my street at the bottom. And again, it depends on what range you're after. You might even do just fine with a cheap TV antenna (not an amplified one): I have a pair of rabbit ears older than me as my backup antenna when it storms and I disconnect everything that goes outside.
