Reception will be highly dependent on your location, especially in the 700-900 bands. The 700-900 bands are FAR less "forgiving" with signal propagation, as compared to VHF or UHF. Frankly I get great reception with a radio shack 800 MHz whip antenna on my 396XT, that's all you really need. If you are getting a weak DTRS signal from your particular location, huge antennas won't help much.
I would suggest compiling a list of all DTRS towers in your county and surrounding counties, and then carefully verify which sites you can hear. You should NOT simply "assume" that you can or can't hear certain sites simply because they are two counties away. You may be surprised which sites you can, or can't, hear from various locations. For example, there are certain high-elevation spots in Broomfield where I can hear DTRS towers from Longmont, Gunbarrel, Bald North and I believe Horsetooth Mountain.
Also, with DTRS when you pick up a control channel you have to make sure that you are truly hearing the site you THINK you are hearing. DTRS uses "duplicate" frequencies for various sites and sometimes you will pick up a control channel at site B 45 miles away, when you think you are hearing site A 20 miles away.
You should understand that radios used by actual DTRS users automatically select the site with the best signal properties, so users don't need to know which site they are using. Moreover, DTRS radios don't get confused by "duplicate" site frequencies because they analyze metadata from the Control Channel to confirm which site is being used. However, no scanner has these capabilities (and if it did it would cost several thousand $).
Also, driving and DTRS can get VERY messy. Basically, you have to learn a lot about "general coverage" patterns of all nearby DTRS sites, and figure out which site is best to use at which location. You will find a lot of forum posts about how many DTR sites one should listen to at once, I generally listen to one site at a time. When I travel in the Denver-Metro area, I frequently switch sites on DTRS to find the best signal. In general, if you are new to DTRS you should plan on spending a lot of time trying things out, failing, trying again, etc. DTRS is a huge system and it may seem overwhelming at the beginning.