The choice of penholder for Copperplate style of writing depends upon the nib you are using and whether you are left or right-handed.
The majority of calligraphers use a straight nib, for example the Leonardt Principal EF or a Gillott 303 nib. If you are using a straight nib and are right-handed you want to use an Oblique Penholder. The reason for this is that an oblique penholder helps a right-handed person hold the pen at the correct angle for copperplate writing.
If you are left-handed then you want to use a straight penholder because you arm and hand are already at (or near) the correct angle. Saying that, a minority of right-handed calligraphers prefer to use a straight penholder with a straight nib and have the paper at a steep angle to help them achieve the correct pen angle.
The Speedball Oblique penholder is suitable for most pointed nibs. Sometimes though, using a pair of pliers, it may be necessary to slightly flatten the part of the nib that fits in the penholder to get a better fit. Scribblers Penholders