UltraDropDown and UltraCombo derive from the same base class, so they support nearly identical functionality. 

To do "StartsWith" Filtering when the user types in the cell, you need to set properties on the grid column (not the UltraCombo/UltraDropDown)

band.Columns["My Column"].AutoCompleteMode = Infragistics.Win.AutoCompleteMode.SuggestAppend;
band.Columns["My Column"].AutoSuggestFilterMode = AutoSuggestFilterMode.StartsWith;

The only thing you can do with UltraCombo that you can't do with UltraDropDown are: 

1) DataFiltering (which is not the same as AutoSuggest like you described here)

2) Editor Buttons (which you probably don't need)

3) CheckBoxSettings (to check/select multiple items on the list at once). 

Although the UltraDropDown control does seem to behave better with respect to resizing itself within a grid cell, it does not seem to have the same functionality that the UltraCombo offers. For one, If I start typing inside the control, it seems to search only as a "Starts with" and there does not seem to be a way to specify a "Contains" search type (as in the UltraCombo AutoComplete and AutoSuggestFilterMode properties). In my scenario we display our records as Code - Description, like:

1001 - Red article

1002 - Blue article 

1003 - Violet cube

So if I begin typing "art" I expect the list to show only  the "1001 - Red article" and "1002 - Blue article" entries. In this example the UltraDropdown just does not finds anything.

Which brings me to my second point: The ultradropdown doesn't seem to filter the way the UltraCombo does, it just seems to position the user at an index inside the list

Until now, only the UltraCombo control does what I want, the only issue is the resizing. Is there a a workaround so that the UltraCombo resizes itself in the same way that the UltraDropdown does?

We need it to be an UltraCombo because we need to display the value selection as a multicolumn grid. Isn't there a way to make it work with the UltraCombo control?