Im comparing between 2 meters, the HP3468A and the HP3478A. Both are 5.5 digit meters, in fact the only difference I can tell is that the HP3478A seems to have on step lower range on the measurements.
Is there any other practical difference between these meters? They are both pretty decent price on eBay.

 

The differences are very small. Mr modemhead highlights here that the 3478A has slightly better accuracy and a 30mV range.
These meters go for quite a good price on eBay - most people despise their somewhat hard-to-read LCD displays in favor of VFDs or LEDs. Personally to me they are absolutely fine meters.

 

I currently have several smaller meters, but they pretty much do not agree with each other. I was thinking of using the HP meter to get a handle on the other meters and see if I can tell which ones are the more accurate one. Both the HP meters are 5.5 digits, which is pretty significant and should allow me to get good accuracy for checking the other meters. I don't really need the super low ranges for volts/ohms that the HP3475 offers, so may go with the HP3468a instead, it's a bit cheaper. Appears to have the same accuracy.

 

That is a good alternative; however, you can never know if the higher precision meter is, in fact, accurate.

If you intend to keep the meters in check, a popular product is the DMMCheck Plus, available from:
http://www.voltagestandard.com/Home_Page.html

 

Yes, I contacted him but he's out until October.

 

I saw that. Well, if you are after doing a basic DC accuracy test, you can always try to get one of those KKMoon multi-voltage references from eBay.
An interesting article is shown at: https://www.markhennessy.co.uk/ad584_references/

 

I ordered and recieved one already, but it didn't some with any calibration measurements, just sais 2.500, 5.000, 7.500 and 10.000.
When I tried it on my meter, all were reading high by about 2 mv or so.

 

Well, if the reading was consistently 2mV above in all measurements, you have a good margin to evaluate the accuracy of your meter.

If you have ordered from eBay and the listing has a photograph with the calibration data (like this one), I would try to return it to the seller. After all, the calibration data is the most expensive part of such modules.

Actually, I just stumbled upon this other module that says it is calibrated with a 6-1/2 digit meter (Keithley 2000) and also has four resistors. Perhaps that is a better alternative?