For my $.02, I'd say that you should go ahead and upres the larger output to 300ppi and output sharpen that. If you were above 180ppi I'd recommend leaving it. If you are upresing anyway, you might as well pick a resolution that requires no interpolation within the printer. I don't think it is overwhelimingly significant, but HP's thermal heads are based on 300ppi frequency, vs. Epson and piezo heads multiples of 360. Some of the variables, and your likelihood of being pleased with the results will depend on the nature of the image. Crunchy, high frequency images with a lot of fine detail will be less forgiving than softer and more subtle images. You are right on the bubble with the smaller print size, and will most likely be just fine without altering the file size. I use Genuine Fractals, and would make a judgement call on it based on the type of image and quality of the file for the smaller print.