What it boils down to is that if you are primarily going to create and work with your documents on the iPad, and do not need the fancier features in Excel, then Numbers will be best. If you need to move documents between Excel on a computer and the iPad a lot, and get a few other (but hardly all) Excel features, Documents to Go seems to be the favorite with QuickOffice coming in a close second. None of the apps are 100% compatible with Excel.
This is not my opinion of the apps themselves, but my impression from reading many reviews and posts in this forum and elsewhere.
From that point on it's all up to you to decide where your work needs are, and research what app will support those needs best. Unless you are under pressure to get something now, you can take your time reading here and elsewhere (like the developer's websites) before making up your mind.
Good luck.