I'm not even going to begin to start trying to quote posts in this thread, some quite random stuff being post, first of all there isn't really any argument that the 3 Series is an 'Executive' car - it's in the Compact Executive class, end of really, no idea what the 'Professional' class is.
Instead, I shall ramble about the 3 Series with the intention of giving a general overview of the range.
Firstly, engines.
The N52 engine is found in 2005-2007 3 Series models. It is available as a 330i or a 325i.
The 325i has 214bhp and will accelerate from 0-60 in 6.8 seconds.
The 330i has 258bhp and will accelerate from 0-60 in 6.1 seconds. There is also a 272bhp version of the 330i which was available for a short period of time in 2007 before the engine was replaced by the N53. It develops 272bhp and will accelerate from 0-60 in 5.9 seconds. It's pretty rare - most of them are 56 and 07 plate Coupes. Not even sure if it made it into the Saloon.
The N52 engine is a dual VANOS Valvetronic engine. It does NOT feature direct injection or a high pressure fuel pump. It is generally regarded as a reasonably reliable engine.
Fuel economy is slightly better, but not hugely, than the old M54 in the E46. I beleive, off the top of my head, the combined is circa 32mpg.
It is only found in pre LCI Saloons - all LCI Saloons do not use this engine.
Next we have the N53 engine.
This was introduced in 2007. It is a completely new engine and the main improvement over the N52 is the addition of direct injection. Power outputs are 214bhp for the 325i (Which is now a 3.0) and 272bhp for the 330i. Performance and power output remain the same.
Fuel economy, however, is much improved. It is this engine which posts the ludicrous near 40mpg combined consumption figures. On paper, it is a clearly superior engine.
HOWEVER..
It has been plagued with reliability issues surrounding the fuel injection system, coils, etc. There are numerous complaints of problems - many quite serious - which far outnumber the issues reported with the N52.
As a result I would strongly caution against running an N53 engined car unless you have a comprehensive BMW warranty and are able to renew this during your ownership of the car.
Finally we have the N54 engine. This is the twin turbo 3 litre straight six found in the 335i. It is a steel block engine not an alu block like the two NASP options - it's based actually on the older M54, but with the addition of direct injection and twin turbocharging. It develops 306bhp and was introduced in late 2006.
This engine also has quite serious reliability issues around its injection system, and is notorious for constant failure of the high pressure fuel pump. BMW North America have issued a recall on this engine to try and address issues with the HPFP. There is no such recall in the UK.
My advice on this one remains as it does with the N53. BMW warranty or do not bother - it really is as simple as that.
So, there we go. Thats the engines.
325i or 330i?
In the past, I've been quite vocal about how pointless anything that isnt a 330i is. Why bother buying a car like a 325i which has the same running costs as a 330i but without the performance? However this applies to the E46, where you cannot move in Tesco's carpark without falling over a 330CI. There are many of them out there and finding a nice one is, or was until they all got old and bought by knuckledraggers, easy.
This is not the case with the E90. The 325i now significantly outnumbers the 330i for completely stupid reasons I will have an anuerism if I go into explaining again. Therefore not only is it much, much easier to find a 325i, they are also a useful amount more expensive. This is quite crucial with a budget of £15k, which quite easily buys a rather nice 325i but only a much older or more ropey 330i.
The performance of the 325i is also not bad anymore, it isnt far off that of the old E46 330i. So, its no longer as clear cut.
Running costs.
No warranty? Forget anything that isnt an N52, and even then, think really hard about whether its sensible. Even engine aside there is a myriad of stuff that can break on these and none of is is particularly cheap. If your CCC (IDrive) goes, it's a 4 figure bill, for example.
Warranty?
Frankly they are all much of a muchness (Assuming we are talking Efficient Dynamics, so September 07 onwards 335i). The servicing costs are the same, they take the same tyres. The only big difference is fuel economy, the 325i/330i are circa 20% more economical than a 335i.
The main difference is purchase price. Like for like a 335i should be about £2k more than a 330i, and a similar difference exists between 325i and 330i. So with a 15k budget you are into the suspect 335i's or the big miles ones - something I recommend against. So I'd discount it for this reason.
There we go, an overview of the E90 range.
You can now return to tearing chunks off each other about whatever it was you were fighting about.