The Q300 is OK, but was produced in the period when Technics as forced to decrease the quality of their products.
I have to disagree and disagree strongly with your statement. There is nothing low quality about the Q300. I have the SL-QD3 which was the model produced right after the Q300 and it has performed well for 25 years.
I will admit none of these tables will win awards nor compete with tables costing much more. But their specifications are such that they are perfectly capable of producing good sounds and much enjoyment for their owners.
Now to give my opinion to the OPs original question, which one SL-D2 or SL-Q300.
My answer is it depends. To me, it would depend on the condition of the tables, and the price. I would be happy with either one. The advantages of the SL-Q300 is simple setup, if you like or can tolerate P-mount cartridges. You don't have to worry with alignment, VTF, VTA or anti-skating. The table has quartz direct-drive and has slightly better wow/flutter and rumble specs than the SL-D2, but only slightly. It is fully automatic, which is an advantage if you like fully automatic tables. It's a disadvantage if you don't.
The SL-D2 uses a standard headshell and is semi-automatic. It gives the owner more options for cartridges and also more options on setup. You can play with VTF, anti-skating and alignment. That can be a plus for those who like to fine-tune their table to match their favorite cartridge. Of course, it is a disadvantage if you are not handy at setting up a table.
As far as sound quality goes, if you have equal cartridges on both, there should not be much difference in the performance. They both are good tables.