A well ridden properly set up CRF150F on the right track can run with 250/450s all day long.
I have 2 XR100s and a CRF150F set up for flattrack and supermoto and they are a blast. Am I going to win a national with them...no. But I have ridden just about everything at one time or another and I have the most fun racing minis.
I'd rather ride a slow bike fast than a fast bike slow.
Buy street tires for your 150 and leave it stock. You can go surprisingly fast on these with little set up. You just need to practice your technique. You can spend a TON on these things (ask me how I know) and go faster, but they are most fun in stock form.