[Sony Ericsson Xperia Mini, Mini Pro, Xperia Pro, A] pengipete:
Ask your sis if there are any features or apps that she really wants because there are a couple of major differences between the Ace and the SE phones - most notably, the later Xperias are already running Android Gingerbread and will soon be updated to Ice Cream Sandwich (the latest version of Android) and they support Flash - which the Ace doesn't. In case you don't know, without Flash she will be unable to use a lot of really good sites and content - such as online TV. It is possible to hack the Ace to use an older version of Flash but it's not very good and certainly not good enough for anything like watching TV.

The Ace is a decent iphone-look-a-like - the SE phones don't copy that styling. That's a plus-point for me - I love the SK17i for being different from everyone else's phone.

The Ace has a good sized screen - but the resolution is no greater and it's not a particularly good quality screen - TFT and sometimes a bit lacking in contrast and image quality. The SE phones have superb picture quality but the smaller screens (like the mini and mini pro) may be off putting for some people.

I had a Sammy and the Xperia Mini Pro in quick succession (cos the Samsung packed in after less than a week) and they are both good but the Xperia is a much better phone for the money - looks good, sounds great, superb processor (matches many dual-core in real-world use), terrific picture quality, solid build, excellent ongoing support from SE, full Flash support, ICS on the way.

Going to Android market highlighted the additional features of the SE phones - very few apps say "not suitable for your phone" with the SE thanks to the Flash support and fast processor.

The only major criticism I have of the Xperias is that the cameras are terrible - either through hardware or software, they take all-but unusable pictures considering the claimed resolution - massively over-processed and appallingly low quality in anything other than bright light. Some people have reported sound interference when recording videos on some Xperias - that has supposedly been cured on newer phones but might be worth reading around if video recording is important to your sister.

SE's user forums are very good - plenty of input from SE themselves. Also, they are well laid out and easy to use. Samsung's are a bit of a mess - hard to find anything. SE's product pages are also very good - plenty of genuinely useful information and videos to help you find your way around the phone - Samsung's site is more like a big advert with little practical detail or advice.

One other possible negative for the SE phones - depending on your situation. SE use non-standard headphone sockets which annoys some people. They are usually fine with normal headphones but won't always work with headsets that have built-in microphones.

Basic apps on both Samsung and SE are decent. Both include some junk but I'd say that SE have less garbage that can't be removed (though that may vary of you buy from a service provider). Overall though, since these are android phones, it's so easy to add apps that it's not a big deal.

SE's support software is miles better than Samsungs. Samsung's software is bug-ridden and unstable - SE's is easy to use and reliable offering some nice features like the ability to quickly re-install the same software if the phone play up or is getting sluggish.

I really liked the main screen layout on the Samsung - I'm not so keen on the SE screen with those corners (really just folders that you are forced to use if you don't want to lose real-estate - I wish there was a simple way to switch them off). Even so, it's easy to install and use GO or one of the other launchers to get around any such niggles.

The Samsungs have one feature I really miss - proper controls in the pull-down menu to directly switch features like wi-fi without going into menus. There are a few apps available for the SE's to mimic that but they mostly just provide a shortcut to the menus so are nowhere near as useful.

Samsung's task manager is easy to use. SE don't really have but they claim that their software handles memory so well that the user doesn't need to do it manually.

Samsungs DNLA support is excellent and simple thanks to their ready-made applications - just install "All Share" on your PC and it's all but automatic. SE phones take more work to get anywhere near that stage.

There seems to be a lot more happening with the Samsungs when it comes to custom roms and mods. That may attract some people - others may see it as evidence that more SE owners are happy with the phone as it is. To me, it seems to be largely down to the fact that the SE's already have the later version of Android making many of the tweaks unneccesary. Also, just based on reading through the XDA forums, it appears that it is easier to recover the SE phones if something goes wrong when tweaking - a hell of a lot of people are bricking their Samsungs because it's all too easy to lose the imei data when tinkering with them. On the SE phones, most phones are "cured" simply by using the SE support software to reflash the phone (probably not a big deal for your sister but nice to know that it's so easy to repair a corrupted phone if something goes wrong)

Last point - the males who have seen my Mini Pro think it looks odd because they all have iphone-clones and no imagination - the girls all love it for the size and looks (I'm talking the phone :))