I will be traveling by train from London to the Cotswolds . I know it's a short trip but , should I do first class ? I'll have luggage and want a sightseeing 2 hours , relaxing !

I'd start by asking where to where (neither London not the Cotswolds is a station).
Many trains offer reservations (free) which gets you space for your self and many trains have a lot of space for baggage, but it depends how much you have. First class always gets you more space and on some lines things like free tea and coffee.

So where to where?

The luggage issue is identical First or Second, the scenery is identical First or Second, the journey time is identical First or Second. You make the call.

Where are you traveling to in the Cotswolds -- there are few statins (Moreton-in-Marsh, and Charlbury or Kingham which are just outside the Cotswolds proper)

Sure hope you don't have boat load of luggage.

The scenery issue can be affected by class on some trains, I suppose that's why the OP asked. It isn't just that you are going on the same route, in some cars, the seating and windows are different. For example, on some French TERs, the first class is one end of a certain car that is higher than the second class, so you get better views. That's unusual on TERs, though. I know on some special Swiss trains, I think there are special sightseeing cars/classes, but I forget the details. On some other French trains, there are two levels and the upper level might have better sightseeing. I don't know if that is a class issue or not as I don't care about that issue. I've been in some trains where I think you get a lot better view in first class as the seats are farther apart and stuff like that.

I have no idea about the particular train in question, though, not having taken it.

I wouldn't expect much nice scenery but happy to be proven wrong on that. I don't know the exact route used.

As others have said, no double decker trains in England but you have more window 'real estate' in first class so you can generally expect better viewing of any scenery that you want to see.

Luggage is easier in first class because generally it's less cramped and you have more room to move around with your luggage, get it up in overhead storage, more room per person to store it in the racks at the ends of the carriage. More room to navigate generally for yourself and your bags. Fewer people per square metre just means more storage space per person. Those who say dealing with luggage is the same in first and regular class must never have dragged big suitcases down a cramped and heaving second class aisle with big people hanging halfway in the aisles.

If the train is not busy then there is much less advantage to first class but when it's heaving, it's very nice.

Strictly a matter of budget vs comfort desires. If you're concerned and you have the budget, spring for First.

Don't expect first to be a big deal -- it is a short trip, though the seats are larger and comfier. We still don't know which station so the route is unknown which can make a difference re equipment.