It also depends on what size tank you are wanting to cool. I do not think drop ins don't come higher than 1/2 HP, so you may not be able to use one depending on your total system volume. Drop in chiller elements take up a good bit of room in the sump, so if you have space issues r/t other equipment in your sump, an in-line might be a better application for you.
Whether you upsize the chiller capacity should really be r/t your current and future needs rather than just a blanket "do it" type thing. I have a 100 gallon reef tank with LEDs and a cool basement location. I don't have a huge temperature pull down requirement, so a 1/10 HP in-line works well for me. This same reef tank on the first/second floor of a house and with metal halides would probably need a 1/4 HP to do the same thing, so location and heat producing peripheral equipment play a big part in sizing your chiller.
Upsizing is not a bad thing necessarily. An oversized chiller would run less, but it could also overload a circuit that a smaller, appropriately chiller would not, depending if you have the ability to provide a dedicated circuit for a chiller or not, which is a good idea, IMO. Upsizing may also be good if you think you will go to a larger tank size in the future, which can eliminate double buying equipment.
The price jump from one HP chiller to another is not bad as well, like the increase of going for a 1/3 to a 1/2, or 1/2 to 1 HP. Not too many 3/4 HP chillers out there any more.
