your personal requirements will decide before you..
as an example I was having serious parking problems and high fuel costs..
as I hate diesels (with any of their expensive parts prices here, you can buy a miggy in UK ) and
fed up of lpg services incompetence (had an lpg car in the past and visited them frequently), I buy a small car with low fuel consumption for daily use.. and I kept miggy..
both without lpg.. (have various reasons I wont go in detail here but not for boot space)..
now for your decision; if your yearly mileage is high your budget requires some low cost car..
either diesel or lpg.. in my opinion the diesel must be low mileage if you dont want to spend extra for repairs after you buy it..which means you will pay your 2-3 years fuel costs just at the beginning.. lpg decision depends your and friends mechanical abilites in which you are surrounded by oofers so you are not alone.. but I cant say lpg installation will be flawless.. anyway you/friends will spend some time for it..and lastly I must say lpg installation must be done on a healthy car otherwise lpg will magnify/complicate the problems and you will be stressed..