What is a trade paperback? What is an omnibus? Which should I get?

Both of these are the physical books that you find at your book stores, they always contain multiple issues, more often than not of multiple series, that tell one specific story arc or event. These will be contained stories that can often be consumed on their own along with some basic knowledge of the characters involved. 

The difference in a trade paperback (referred to as "trade" from here on) and an omnibus is what it contains. The trade will usually only contain the issues that are absolutely necessary to enjoy the event that it is about and you might get some concept art in the back of characters or issue covers. The omnibus will always have more content than your average trade paperback. The omnibus will either contain a large selection of a legacy title and be broken up into multiple omnibuses, or be centered around an event. When they are centered around an event they will often have issues from other series that show the impact on the larger world and issues from the series involved slightly before and slightly after the event to give extra context that the trade does not provide. 

These often range in price depending on the quality of them. Your average trade paperback  will be the price of the single issues that it includes, while your collector's edition versions or hardcovers can often get a little more pricey but for a higher quality. The omnibus will invariably be more expensive than the trade but that is due to the larger volume of content within the omnibus, as mentioned above.  The main reason to collect these are for collection's sake. Some people prefer the look of a larger book on their shelf than having a large assortment of individual issues. At the end of it all it is ultimately up to preference. As for which one you should get that is also up to preference. Personally, if I am trying to collect a large legacy title usually an omnibus is easier to collect than small trades or single issues. However, for events I almost always get the trades, they give me exactly what I need on a reread which is just the event without all the extra issues for context that I already know. 

EXAMPLES

Batman: Year One Trade Paperback

Avengers vs X-Men Trade Hardcover

Invincible Omnibuses