Either!

Disclaimer, I do not use water. Others use 91% alcohol. Some even use distilled water. It's an individual choice. I prefer the cheap 70% alcohol. As for how long a bottle lasts is depending on how quickly you apply the glue, allowing the alcohol to not evaporate too much. In my particular situation, I fear water because if you use too much, it could cause problems. When I spray my alcohol, I use enough to soak the ballast only, and apply the glue asap. I would guess that using water would give you more time between spraying it and actually applying the glue. Another thought I have. First of all, I'm frugal. Hate to spend money when not necessary. But I chose the alcohol because I had a good supply. Now that COVID has creeped in our society, the cost of alcohol has gone up. So, for you, if you think that saving those bucks is important, go ahead with water. OK?

One thing

If you use dish soap for the wetting agent make sure it is clear..........don't use Dawn (it's blue). 

Stick with the alcohol.

Water

I have used water and dish soap, and a mix of alcohol and water. Both worked. I don’t use straight alcohol because the vapors irritate me.  I have had some alcohol react with white glue , making it gooey.

50:50

I too used water and dish soap, for many years. 

Now I prefer a 50:50 mix of water and alcohol.

Cheers,

Steve

Either

I use both “wet” water and alcohol (70%).  To make the water I have always used Dawn, and never had a color issue.  I use 4-5 drops per half gallon; I suppose perhaps if you use too much it might color the water?  Never seen that, though.

I have uses watered down

I have uses watered down white glue in a cup with a drop of dish soap for decades with great results. I also mist the area with diluted alcohol before applying the glue.

Dawn is fine

I use water with just one drop of Dawn in a small sprayer. Works fine as a wetting agent and one small drop of blue Dawn does not color the water. . 

I guess I used too much

It was a strange experience. After carefully ballasting a long section of the mainline I let it sit for a few days while I worked on other tasks. Every time I entered the layout room something was bothering me. The ballast was dry but it just didn't look quite like I had intended. I finally realized that it had a light color shift towards blue. Damn! I then remembered that a few years before a good friend showed me his latest ballasting effort that he was getting ready to completely tear out and re-do. Here's another "don't do". Black Sharpie markings of track centers, over written with revisions and other notes. It will disappear when the ballast goes down....    right?........not in this case. If a little black comes through....no big deal. Problem is once it gets reactivated with alcohol it turns a strong purple.

In all fairness, I am extremely sensitive to blue hues where they don't belong. it's a bit of a curse. Don't even get me started about an inkjet printer not being able to get a neutral gray when you are trying to put together a photo-lam structure with a large proportion of gray in the offing.

 OK, I feel better now.

I have been doing tests on

I have been doing tests on glue ballasting samples looking for the best clearest glue and most flexible sound deadening glue it is difficult to have both. But @Rick I came the next day and one of the glues had grown a weed like fungus about 1/8 inch high.

Depends

I used to use isopropyl alcohol for nearly everything.  I've encountered a few materials that don't work well with it, but are fine with water and dish soap (e.g. Scenic Express ballast).  Since nothing I use for scenery has a problem with water and dish soap, I switched to using that exclusively.

colored dish soap

I haven't looked specifically, but can you even get clear uncolored dish soap? Everything I recall seeing is some color or other, the stuff on my kitchen sink right now is orange. I know the most popular brand in my area is pine green. 

If you have to have colored dish soap, and a slight color shift, what color is best? Maybe my orange won't be as bad as blue or green?

Colored soap

If you are using enough soap that it colors the ballast you are using waaaaaaaaaaaaaaaaay too much soap.  We are talking about drops per quart of water.

Exactly..

Dave. Here's what I do. The alcohol is 70% in a pint bottle. I use 2 drops/no more. As for color. I found some clear dish soap at Dollar Tree. And I assure you, it's enough to last till Hxxx freezes over.I would have to assume that 2 drops in a pint bottle of water would be sufficient. It's just enough to break the tension mentioned in the first post by Steve Klein. And FYI, here's a picture.

The bottle marked MP+alc./Mod Podge is a pint bottle from alcohol. Has a very small hole below the cap that I can simply hold the bottle, let the mix drip/dribble as I move along. Very easy. I believe in minimum fuss and all this requires no more than a wipe off the top when done. No cleaning up. No more pipettes or eye droppers. Too much extra work. FYI the paint brush is used to spread ballast, the toothbrush to clean turnout points, etc. Hope this helps!

2 drops

Well I would have assumed that the color wouldn't make a difference, but someone posted a warning about it, so I asked.