Check the final evolutions (because this is what ultimately matters), then the respective Pokemon's abilities, movepool, and stats.
We'll use Ralts vs. Abra as an example. This is effectively Gardevoir vs. Alakazam, since those are the final evolved forms. Look at their Smogon analysis pages. Here is the one for Gardevoir and here is the one for Alakazam (for Ruby/Sapphire/Emerald).
We can immediately note some differences:
- They both have a very strong Special Attack state, but Alakazam is slightly stronger.
- More importantly, Alakazam is fast. It has base 120 Speed, which puts it well above most Pokemon. Gardevoir's 80 speed is pathetic in comparison, and not enough for sweeping* without boosts.
- Both Alakazam and Gardevoir are relatively fragile, but Alakazam is significantly more fragile since it HP and defensive stats are lower. Gardevoir's 115 Special Defense is very respectable, so it can sponge Special hits.
- Alakazam gets Synchronize & Inner Focus as its abilities, while Gardevoir gets Synchronize & Trace. In other words, Gardevoir's abilities are more useful. Inner Focus is effectively a null ability, while Synchronize is relatively weak as well. Trace is not outstanding, but it's more useful: you can e.g. Trace Levitate for an extra immunity.
And then you can look through their movepools. They share most moves, but there are some noticeable differences. Here it's important to be familiar with the "meta" and know which moves are worth using. I don't know R/S well, but based on my experience with other generations, these moves are notable (the analysis pages agree, since these moves get used in the movesets available):
- Alakazam gets Encore and Recover. Encore is a niche move that helps your team set up, while Recover gives Alakazam longevity.
- Gardevoir gets Will-O-Wisp, Wish, Memento, and Hypnosis. Will-O-Wisp means it can act as a check to slower physical attackers. Wish and Memento are strong support moves. Hypnosis is less strong, but it's still a sleep-inducing move that you can use to set up.
These differences therefore lead to the ultimate answer to your question:
- Alakazam is a fast offensive attacker. You will usually give him offensive EVs (i.e. in Special Attack and Speed). He will hit hard, but he will usually not be able to take much damage. Like a firefly, he will burn brightly, but only for a short while.
- Gardevoir is more of a support Pokemon. You use her to make your other Pokemon shine. She will be more defensive, with EVs in defensive stats. She will be usually able to take a hit. However, offensively she is likely to be lacking not just because you won't have the space for support moves on sweepers, but also because you will not be investing in Special Attack or Speed.
Ultimately which Pokemon you use will depend on what role you need it to fulfil in your team.
Pedantic sidenote: both Alakazam and Gardevoir are rated in the UUBL tier for R/S at Smogon. If they are not in the same tier, then a heuristic is that the Pokemon that is in the higher tier is the "better" Pokemon - but it is only a heuristic; it is possible the lower-tier Pokemon is more appropriate for your team.
*A sweeper is a Pokemon whose purpose is to hit hard and knock out the other team's Pokemon. It is distinct from walls and supports (who do things like set up entry hazards and absorb hits).