And the subject talks are on the Tuesday arent they?
EDIT: Ive just had a look at the orientation brochure and according to that the psychology subject talk is in the physics building, lecture theatre A at 11.30 on Tuesday, is that what you mean by a science talk because it is in the physics building? If it is then dont let that worry you, that will be a general psychology talk aimed at both BSc and MA students (as there will be a lot of similaritys between the courses) and it is only in the Physics building because it is a large building that is easy to find and has nothing to do nescicarily with the nature of the subject. If you look at for example the Linguistis subject talk is held in the Computer Science building (John Honey) and Management is in the Chemistry building (Purdie) do not think that just because psychology is in the physics building it will not be relevant to you, its probably just the best place they could put it in freshers week when most buildings are closed.
EDIT 2: Ahhhh you mean the welcome, dont worry about it they are both pretty much the same apart from in one the Dean of Arts is on stage along with everyone else to welcome you and in the other the Dean of Science is on the stage with everyone else to welcome you. You just get a brief introduction to the University and a hello from the big wigs (Including either the Arts / Sciences dean) told about the Union an about various welfare services and a few safety warning. They do it in arts then sciences for the simple fact that Younger hall although big is not big enough for the whole first year at the same time and with fewer people doing sciences than arts they would prefer Psychology which can be either a Science OR an Art go go in with the Science crowd so it doesnt fill up when the Arts students are in. In theory you can go to either as there in no checking of what your doing or who you are at the door you just sit down and get bored to death for an hour. Really dont be worried about it, if you want to go to it go to either the only difference is who says hello but in my experience you are not missing out if you give yourself a lie in (I only wish I knew that last year). If you want to find out specifically more about your subject go to the Tuesday Subject talk I mentioned above (Also useful for any modules your considering)