Here's a quick text that I hope will explain the different strength level of the same pokemon, I wrote this originally for my Mom, so I hope its easy to understand.
So here we see a bunch of Eevees I caught last week, I have marked each one with good IV stats as favorite.
Marking good IV stats pokemon as Favorite is something I would definitely recommend it has helped a lot for me to save time when I want to mass evolve(during a lucky egg) and so on. It also will help later on when trading comes as for example that 419 Eevee has amazing stats, but I already have a lot of Vapereon (and 1 Jolteon) that I want to spend Eevee Candy on, so I wont likely level it up any time soon. But I still noted it as my assumption is that people are not just gonna trade to get all pokemons but to get those specific high stats ones.
Because its not all about where the pokemon is on the CP scale bar.
For example here are 2 images comparing my 580 with my 572
The 580 would appear to be stronger, as its 8 higher then 572 but if we now look at the 572.
Its CP bar is actually a bit further away form max, this can also been seen at the cost for "power up".
Power up the 572 CP one cost 2500 stardust while the 580 cost 3000, this means the 580 is at least 1 level higher.
The 572 also has higher Health Points.
What this means is that the second here is way better to evolve, it got quite the better stats actually, its power upped it will gain something like 20-30 more cp and 1-2 hp, this will put it at roughly 20 more cp and 6+ more HP. Of course this also scales later for a Vaporeon,or other eeveelutions, but Vaporeon got the best stats, so the effect of good IV create a bigger difference. If they would both be evolved (staying at this CP Curve stat) the second Eevee here can have more then 100CP, mainly becasue of Vaporeons insane base stats, then investing Stardust in them and getting them to max Level they will end up having several hundred difference in CP. On this level(20) when I took these images, a top pristine Vaporeon will have almost 300 more cp then a the weakest ones.
Please note that this 580 one, does not have terrible stats, so the difference is not a clear as it could be.
In this case its quite easy to spot with your eyes right away that the 572 is clearly stronger, otherwise there are several different IV calculators out there that can be used to determine a Pokemons value.
My main point is that just a quick glance can help you in avoiding evolving weak IV Pokemons.
My recommendation is to save your Candy at higher levels (15+) and only evolve actually good pokemons, except for the EXP grind ones like Drowsee, Zubat etc. Already at 20 I had a very strong team as I had saved up e.g 125 Nidoran Candy until I caught a max IV Nidoran.
However, you don't have to farm like crazy and just save up candy, but I would say that just get like 3-4 on a good CP bar level, e.g 80% or above these are the ones you want to evolve, You don't want to spend time evolving a 10% CP one just because it has good IV stats, that will cost to much Stardust to get to 100% and its not worth it by a long shoot.
So after you got those 4 on a good CP % you should be able to compare their HP and CP value on the actual bar, this will tell you who to evolve. Going and doing the tedious calculator sites is something I would say its only needed when you have a hard time finding any pokemon of that type at high CP % but you can still evolve it (maybe from a good candy egg etc).
For example this would be like having caught 50 slowpokes, but only one slowpoke at 80% CP, Slowbro is really strong so I would then go to a site and compare if that specific slowpoke is worth the 50 candies.
This is currently my best Slowpoke, it has pretty high on the CP Bar. It has 616 which looks great, but its actually very weak for my trainer Level, it can have like 750 which will be x2 as a Slowbro at least. So I have not favorite marked this Slowpoke instead I am waiting to get a better one.
But if I had like 200+ candy sure, then I would evolve it just for the exp alone.
