With a lugged and brazed frame the tubes can be separated from their lugs and replaced if they become damaged... a simple brazing setup and replacement tube is all you need and with a standard 1 inch tube size finding a spare should not be hard to do.
A lugged bottom bracket is very strong and if well made should never cause any problems whereas a welded bb may suffer from issues stemming from overheating... as a mechanic I have seen a good number of cracked bottom brackets on welded frames but it is rare to see a lugged frame with these issues.
For the long haul I would pick a lugged frame due to its ease of serviceability in the event of mishap as a person with basic skills would be able to repair it properly whereas repairing a welded frame is a little more difficult.
On many bikes the welds at the chainstay seem to be a very vulnerable point as this is where a lot of load is concentrated... my friend's Kuwahara mtb developed a crack at the rear dropout (a common problem with 1984 - 1985 Kuwaharas) and our local frame builder manufactured and then replaced the stock dropouts with new ones for $100.00.
The bike should be good for another 25 years of riding as it is otherwise still perfect.