Do I need a more aggressive razor or a sharper blade?
Assuming that your face is properly prepared and your lather is right, there are some indicators to help answer this question. If during your shave it feels like the razor blade is not making contact with your hair or you have to go over the same area several times, then the razor is too mild for you. If you feel that the blade is making contact with the hair, but it is tugging or pulling and you have to apply more force to move the blade along, then you need a sharper blade. If your razor blade removes a lot of hair in one pass and you feel that the blade is scratching across your skin, then the razor is too aggressive for you. If you feel burning or scratching in your final passes due to the friction of the blade on the skin or your aftershave burns your face excessively, your blade is too sharp.
In my experience, almost all new wetshavers believe that they have thick beards and sensitive skin. The truth is, however, that most men are in the middle and a medium aggressive razor, such as a 34C or DE89L, is the best place to start to experiment with different brands of blades. In these razors, a Derby will be mild and a Feather will be sharp, so you can use these to learn where your beard type falls on the continuum. Later you can experiment with other combinations of razors and blades. For example, you could try a mild razor like a Feather All Stainless or iKon Bulldog (regular comb), where the blade gap may not expose enough blade so you may need a BIC or Feather blade to cut through your stubble. Or you could try a Derby in your Merkur Slant, to test if the guillotine slicing provides a good combination of cutting the hair with a blade that will not be too harsh on the skin.
There is a balancing act that happens between a safety razor and a double-edge blade. The goal is to find the right combination for your skin/hair types so that you have the blade sharpness necessary to effectively cut your stubble and the cutting head/safety bar that will keep the blade from irritating your skin. Learn more in Safety Razor Aggressiveness and Choosing the Right Blade.