If anyone tell me what this 'gooey' slang word means...
Tyranids and Arachnids is a simple story.
Bugs are space insects. Tyranids are space reptiles (lizardmen).
Both races have some things in common but come from a totaly different approach - just compare the Tyranid Warrior to a Warrior Bug.
I like the bugs more because they don't look like a bunch of radiation-mutated Post-Chenobyl-reptilehumans.
Tyranids still look some kind of human-grown, where a Warrior bug looks totaly alien. You don't even imagine you could talk to this thing.
For the models, I like the bugs more. They are made from a better plastic and have far more details on their skins.
Look at the average gaunt or even the new carnifex and you have plenty of smootly unscructered parts. Now try to find an unstructered skin part on a Warrior Bug - good luck, you'll need it.
Here's a pic to show you what I mean. The Warrior has about 100% more skin detail than the gaunt (who actually doesn't have any at all):
In an War... well, I'd go for the bugs.
Tyranids are intelligent, but not in a degree the bugs are. They don't have this sense of exploiting tactical advantages and their enemies weaknesses.
In addition tyranids depend on evolution - nothing to compete with genetical engineering experts.
And in the comparison of the basic fighting unit the Arachnids clearly win. A Warrior Bug Horde would tear through a Gaunt Pack with ease. Warriors are much bigger, stronger, better armoured and they don't depend on their bigger cousins to tell them what to do.
Also Bugs are much tougher. During the tyranid invasion of Macragge most of the smaller creatures died during the icy nights. Bugs wouldn't care, damn bugs lived on Pluto.
Both of them have infiltration capabilities though I think Controll Bugs and their victims are worse than genestealers and their cults.