I used to have a 300D w/ continental engine, if I didn't pipeline at all I would have definitely kept that machine, it could fab pipe like nobody's business.  You really can't beat those welders on fuel economy and engine longevity.  I still personally like to run full size machines just for the fact that you've got a ton of reserve power if you need to do anything heavy, like gouging or fabbing 16" sch. 80 (filling with 1/4"). I know everybody goes on about the ranger 305's with 100% duty cycle and all that, but I'm still a bit stubborn about buying one.  My old 300D load banked out at 402 amps max. so, with it's 60% duty cycle I could weld at 241 amps for 10 minutes out of 10 minutes (theoretically speaking). And with the full size machines I personally feel it's alot easier to perform minor maintenance and repairs, i.e fan belt, spark plugs(my sa-200), air filters, fuel filters, oil changes, thermostats, etc... I'm sure someone will have a valid argument against my opinions but that's just par for the course, hopefully this helps your decision anyways...

I worked with a fellow who had a 300d kubota and he was pleased with it I think it's a good choice if you want ac power. the only thing I noticed was the kubota engines seem to run cold for a diesel machine. he said he changed his thermostat and it didnt really help so I left mine stock and just blocked the rad with a piece of cardboard for the winter. I have the 200d kubota and after having put 600 hours on it seems to be the best machine I've had so far. I'll tell you after I mainline with it. that's the only thing I havent used it for. I even gouged with it using 3/16 carbons and 4th gear and 100 was all it needed.