The KG-UV9G is an HT (handheld transceiver), not a "mobile" (automotive style) radio. The used commercial Kenwood/Motorola type radios you can find on eBay are often mobile radios, although HTs can be found there as well. Many mobile radios offer higher wattage output than what an HT will deliver. Most HTs will put out 5 watts at the most.
Midland offers mobile radios (like your MXT275) that put out as much as 40 watts (MXT400) or as little as 5 watts (MXT105). Anytone has a small mobile radio (AT-779U) that puts out 20 watts. B-Tech and Wouxon offer 50 watt radios (GMRS-50X1 and KG-1000G, respectively).
A lot of what you decide should be based on how you plan to use the radio. If you'll just be using it to talk to other people in your family while you're out hiking, or while traveling in one car following a family member in another car, then you would likely be fine with just an HT (well actually, a pair of them). If you want to talk to other people while driving to work, for example, or talk over longer distances, then a mobile radio would likely give you an advantage due to both the higher power output and the better antenna that most mobile radios are paired with.
The KG-UV9G is probably the most feature rich GMRS HT available. In the mobile arena, the KG-1000G is probably the most feature rich GMRS mobile radio available. Both of these radios, as well as a number of others, allow you to monitor other frequencies, including some ham frequencies. There are a lot of choices out there, but what would be best will depend on how you want to use it.