I have an old (2007-2008?) sata2 2.5 320GB Scorpio blue 5400rpm on usb 3.0 dock and transfer at about 40-60MBps depends on how full the drive is. Not sure if that gives you enough information on making an estimation of how fast your HDD will run.
I recommend you to search the internet, find out the bench mark for your HDD or take it out and put into a computer to run a benchmark to see how much faster the drive transfer compare with usb 2.0 of about 30MBps and to see if it is worth to get usb 3.0 or not.
But if you have usb 3.0 port, you are very likely to use them for your future storage expansion, so your usb 3.0 enclosure will be reused with a bigger drive and nothing goes to waste.
Buy it now or buy it later, don't make much of a difference.