This depends on your process. Please be aware that adding a grate to your ball mill will reduce your EGL by around 500 - 600mm (the depth of the pulp lifters and grate plate / liner thickness, plus it will also increase your mill motor power draw by anything from 10-20% for the same throughput.

As secondary ball mills are usually fed a relatively (compared to SAG mills) fine F80 and have high re-circulating loads and produce product fineness of the P80 - 100 to 160 micron range, fitting a grate discharge needs to be carefully considered.

An advantage is that the grates will retain more balls than having an over flow; however this can also be achieved via a dam ring on the outlet.

That was used a lot in the past and with time, they have all disappeared. A few remains.
It might be desired only if you have a coarse P80 and you want a sharper particle size distribution but an overflow mill can do the same if properly set (ball size and charge).

Could you provide any more information on dam rings? Are there any other options for keeping the balls in the mills? How about scats? I've seen the trunnion magnets but they're quite expensive. Any inexpensive options out there!

Dam rings are basically a metal disc made out of wear resistant steel with slots or apertures cut in them that are large enough to allow the slurry to flow through, but small enough to keep grinding balls inside the mill, until the scats are small enough to also pass through the slots. Re scats, you have to get these out of the mill at some stage, as these just consume power when they are too small to do any work on the ore. Normally scats are separated out via the trommel screen - they simply fall off the end into a scats hopper, whilst the slurry falls through the trommel panels. If using a screen deck, the same applies.

The OD of the dam ring should be the ID of your mill trunnion, the ID of the dam ring can be 0 (i.e.: fully closed), or you can set it up like a weir, with leaving an opening in the centre. Other options are rubber reversing spirals, these work well when fitted with dam rings, however you need to have a long enough outlet trunnion to fit these.
Trommel magnets work well, but some ores are also slightly magnetic, so not an ideal solution in all cases.

In general, the use of aggressive attitude denotes an intensive (not selective) application of energy, such as "high energy" as suggested by the manufacturers of grinding bodies.

It is important to define the concept of secondary grinding operation mentioned here, because there are many possibilities:

Grinding of what type of ore?
What range of F80 and P80?
We are grinding specific middling’s grains? Or is it the whole ore (ROM)?
It is secondary grinding, prior any concentration or regrind circuit within the concentration?

The mill product goes even to some concentration or is a final product (to the pipeline; or pellet feed, etc) In general, the degree of grinding to try searching the liberation of certain particles rather than merely obtaining a P80.

If a tubular ball mill for grinding is used, then the first option should be open circuit grinding, in overflow type, and low filling balls (20 to 25%), searching for releasing not only reduce in size.

In simple words, grate discharge mills give higher tonnage, but produce coarser grinding, that may cause problem in your Flotation recovery, Grate discharge will also reduce your EGL. The best option would be to place a retainer ring at discharge end to hold more charge & that will give you more throughputs without compromising grind size.