Should you get a bigger scooter? Only you can really answer that but here's some thoughts.
Bigger is not always better. A bigger scooter will be better in some ways, not so good in others. For running around town a 150 is hard to beat. I have four bikes in my garage ranging from my S-8 up to a couple of 650cc motorcycles. For around town I almost always take my Super 8. It's just more fun to ride in town and I'm less likely to get a ticket. Outside of town it's still fun but top speed can sometimes be an issue on faster roads. Mine will typically top out at around 65 on the speedo although I have seen 73 down hill. I occasionally take it on the interstate for a short distance but only in light traffic. I have done a couple of 200+ miles days on the S-8
For longer trips a bigger scooter will work better. I have taken a 2100 mile, 8 state trip on my Aprilia Sport City 250 with my camping gear. 300+ mile days are no problem on the 250 and I have done a 550 mile day. So, a 300 will be plenty big enough to do just about any trip.
BTW, I have ridden both the DT300 and T Max. The T Max had plenty of performance but I just didn't feel that comfortable on it. I really liked the DT300. I'm 6-2 with a 34" Inseam and didn't really feel cramped. I wouldn't hesitate to ride cross country on either Kymco 300 although the larger gas tank on the DT would be better as is the fuel filler not being under the seat.
Here's a link to my test ride on the DT300:
http://bamarides.com/ride/scooters/kymco-dt300-test-ride/Here's my ride report on my 2100 mile trip on my 250:
http://advrider.com/forums/showthread.php?t=803404Keep in mind that the Kymco 300 has about 6 more HP than my Aprilia so the performance is much better.