I can give you some of my personal experience on this topic, but YMMV when it comes to these meds.
I've used Viagra, Cialis, and Levitra over the past 5 years. I've used the chewable viagra, and I'm not sure if that's the exact same thing you're describing as dissolving under your tongue. I'm not a doctor, so I don't know all the technical reasoning, but the effect is that chewable or dissolving meds enter the blood stream through your saliva glands and have a faster effect than regular pills entering the bloodstream via digestion.
I have found that chewable V starts working in about 30 minutes, vs. about 1 hr for the pill. Also, it's a fact that a heavy meal or alcohol adversely affects the V pill, whereas the effect of a full meal is significantly less with chewable. In fact, I've found that eating a full meal after taking chewable did not affect it at all. I'm not a big drinker so I can't really compare the 2 with alcohol.
I haven't used the V pill in quite awhile, and only use the chewable. The overall erectile affect of the 2 are comparable. It actually seems like the chewable may last a little longer for me, up to 5 hrs, though I've not done a timed comparison.
As for cialis, 20 mg is full strength, as 100mg is full strength for viagra. So it would seem that 1/4 V might be comparable to 5mg C. I've also had success with 1/4 V, but never tried 5mg C, I only tried the full 20mg. I usually prefer 1/2 V though, as it will last longer than 1/4.
Differences I've found between Cialis and Viagra:
V- lasts about 4 hrs
C- lasts up to 24 hrs (I know the ads say up to 36hrs, but I've never found it to work longer than 24, max)
V- requires less stimulation to get an erection than C. I've found that less stimulation is required to get an erection with V than with either cialis or levitra. This is true even with 1/4 V compared to 20mg C.
Finally, side affects of both are well documented. While YMMV on the side effects, you experience them mostly while they're in your bloodstream. And so, if you're getting side effects with C, you'll get them for 24 hrs (or more), whereas you get them for about 4 hrs per V.