I currently use the testors black bottle glue. It's a very thin product and is easily applied in small amounts through a very thin steel tube applicator tip. The red twin pack glue is a thick gel that is a bit more unwieldy to apply. It's been years since i've used the thick gel type and I recomend using the black bottle stuff. It's the type of glue that will melt the plastic and create a permanent bond.
Reading the packaging a little closer it looks like the black bottle now uses a plastic applicator tip. This model master version (still Testors) has the metal tip, which I like. https://www.amazon.com/dp/B0006N6ODS/ref=psdc_2236128011_t1_B00005CA7S
Games Workshop has a glue that is the same as the black bottle Testors/model master but the the GW bottle isn't as good and the applicator tube can pull out of the bottle on you if you're not careful.