It's crazy how old games are still compelling - Pac-Man, DK, Joust, Galaga. Admittedly, Space Invaders was something I hadn't played in a long time as it was a little too simplistic, but it was still a buttload of fun. Enter Return of the Invaders (1985) & Space Invaders '95 (1995. duh).
They are updated and snazzier sequels to the classic that manage to add significantly to the game without changing the core. Both have very colorful updated (and sometimes goofy) graphics. Both have imaginative and updated gameplay and both are a blast (pun intended).
Return of the Invaders (above) is a more straightforward game with some great additions. The invaders don't just march down and across, but some circle and hover Galaga-style. Also, the shields that are in front of you that you can hide behind are more complex than the originals. They change color as you hit them and they deteriorate so you know how close they are to collapsing. Also, they have parts that can be hit and split apart to leave openings and separations in the shield. The final difference here is that also like Galaga, you can hit an enemy ship floating across the top that will release a ship it's carrying - if you collect this ship as it falls down, you increase your firepower.
Space Invaders '95 (above) has more details going on. The backgrounds are crazy goofy colorful. The enemies are aliens and animals and food and shoot all kinds of crazy crap at you. Some levels start with a few BIG aliens that break apart into smaller ones when you hit them. There are bonus stages (again, like Galaga) and Boss Stages (they are dominated by one big dude that takes tons of hits, but there are other enemies there, too). You also have a wide variety of bonus stuff and power-ups to collect. You can wind up with better shots and even bombs.
In the end, here is a quick comparison...
Return is much harder. Return is closer to the original. Return has a more serious tone. Return has more enemy flight variation.
'95 is more colorful. '95 has bosses. '95 has scrolling backgrounds. '95 has more surprises. '95 has power-ups. '95 is goofier.
Most importantly, both are lots of fun.