Hi Clawsie...
I realise your question was about sharpness and see you only joined in May...
You don't say what other lenses you have for you 500D, but if you don't have any Canon EF series lenses, don't forget if it's relevant to your low light birthday cake photos, that both lenses are EF lenses and will have a reduced angle of view when attached to a 500D.
I have an EF 50mm f1.8 (most DLSR owners do - $110.00 today online) and rarely use it because of the reduced viewing angle on my 7D but it keeps my iso down when I do use it.
I have been considering for some time an EF 28mm F1.8 USM, EF 35mm F2.0 or EF 50mm F1.4 USM because Canon will probably never release a dedicated EF-S series F1.4 or F1.8 lens at a focal length (31mm) to match the angle of view on a full frame DSLR as is the 5D and 1D.
But... if sharpness first and low light capability secondly are your main prioritys and angle of view doesn't matter, then the info other members have given should help you make your decision.